Patent application title: Handover Method Of Wireless Communication System, And Wireless Base Station And Wireless Terminal Of The Same System

Abstract:

To prevent throughput reduction during handover, the wireless
communication system includes a wireless terminal and a plurality of
wireless base stations communicating wirelessly with the wireless
terminal and performs control to lower the speed at which the wireless
terminal communicates with a communication party upon time-out of a timer
value generated in accordance with the time required for
transmission/reception of a signal between the wireless terminal and a
communication party, wherein the wireless base station performs the
control to lower the speed of the communication with the wireless
terminal so as to increase the timer value when the wireless base station
decides execution of the handover of the communication with the wireless
terminal to another wireless base station autonomously or in response to
a request from the wireless terminal, and subsequently, executes the
handover.

Claims:

1. A handover method of a wireless communication system, the wireless
communication system including a wireless terminal and a plurality of
wireless base stations which communicate wirelessly with the wireless
terminal and performing control to lower the speed at which the wireless
terminal communicates with a communication party upon the time-out of a
timer value generated in accordance with the time required for
transmission/reception of a signal between the wireless terminal and a
communication party, whereinthe wireless base stationperforms the control
to lower the speed at which the wireless terminal communicates with a
communication party so as to increase the timer value when the wireless
base station decides execution of the handover of the communication with
the wireless terminal to another wireless base station autonomously or in
response to a request from the wireless terminal, and
subsequently,executes the handover.

2. The handover method according to claim 1, whereinthe wireless terminal
transmits a request for handover execution, and,when the wireless base
station receives the execution request, the wireless base station decides
the handover execution and transmits an enabling acknowledgement of the
handover to the wireless terminal.

3. The handover method according to claim 1, whereinthe wireless base
stationjudges whether the handover can be executed or not on the basis of
information about the execution condition of the handover, and,when
judging that it can be executed, decides the handover execution and
transmits an enabling acknowledgement of the handover to the wireless
terminal.

4. The handover method according to claim 1, whereinthe wireless base
stationacquires the present timer value from the wireless
terminal,estimates the time required for the handover, andperforms
control to lower the communication speed on the condition that the
present timer value is smaller than the estimated time at the time of the
handover-execution decision.

5. The handover method according to claim 1, whereinthe wireless base
stationrecords history information about the communication speed at the
time of generation of the timer value or acquisition thereof from the
wireless terminal,calculates the lowering value of the
communication-speed lowering control on the basis of the history
information, andperforms control to lower the communication speed by the
lowering value.

6. The handover method according to claim 4, whereinthe wireless base
stationreacquires the present timer value from the wireless terminal
while performing control to lower the communication speed, andperforms
control to lower the communication speed until the estimated time falls
below the reacquired, present timer value.

7. The handover method according to claim 2, whereinthe wireless terminal
postpones the handover when the wireless terminal does not receive the
enabling acknowledgement of the handover from the wireless base station
with respect to the request for handover execution within a predetermined
period of time.

8. A wireless base station of a wireless communication system, the
wireless communication system including a wireless terminal and a
plurality of the wireless base stations which communicate wirelessly with
the wireless terminal and performing control to lower the speed at which
the wireless terminal communicates with a communication party upon the
time-out of a timer value generated in accordance with the time required
for transmission/reception of a signal between the wireless terminal and
a communication party, the wireless base station comprising:handover
decision means which decides execution of the handover of the
communication with the wireless terminal to another wireless base station
autonomously or in response to a request from the wireless
terminal;communication-speed control means which increases the timer
value by performing control to lower the speed at which the wireless base
station communicates with the wireless terminal when the handover
decision means decides execution of the handover; andhandover execution
means which executes the handover after the control to lower the
communication speed performed by the communication-speed control means.

9. The wireless base station of the wireless communication system,
according to claim 8, whereinthe handover decision meanscomprises a
handover-execution-request receiver which receives the handover execution
request from the wireless terminal anddecides execution of the handover
when the handover-execution-request receiver receives the request for
execution.

10. The wireless base station of the wireless communication system,
according to claim 8, whereinthe handover decision meanscomprises an
execution-condition storage which stores information about the execution
condition of the handover and a handover judgment unit which judges
whether the handover can be executed or not on the basis of the
information stored in the execution-condition storage, anddecides
execution of the handover when the handover judgment unit judges that the
handover can be executed.

11. The wireless base station of the wireless communication system,
according to claim 8, whereinthe communication-speed control means
comprises:a timer-value acquisition unit which acquires the present timer
value from the wireless terminal;a handover-time estimator which
estimates handover time, which is time required for the handover; anda
handover-time judgment unit which judges whether the present timer value
is smaller than the handover time estimated by the handover-time
estimator at the time of the handover-execution decision performed by the
handover decision means, andperforms control to lower the communication
speed on the condition that the handover-time judgment unit judges that
the present timer value is smaller than the handover time estimated by
the handover-time estimator.

12. The wireless base station of the wireless communication system,
according to claim 8, whereinthe communication-speed control means
comprises:a history record unit which records history information about
the communication speed at the time of generation of the timer value or
acquisition thereof from the wireless terminal; anda lowering-value
calculator which calculates the lowering value of the communication-speed
lowering control on the basis of the history information recorded in the
history record unit, andperforms control to lower the communication speed
by the lowering value calculated by the lowering-value calculator.

13. The wireless base station of the wireless communication system,
according to claim 11, whereinthe communication-speed control means
comprisesa repeat controller which repeatedly performs, while performing
control to lower the communication speed, acquisition of the present
timer value by the timer-value acquisition unit, estimation by the
handover-time estimator, and judgment by the handover-time judgment unit,
until the handover-time judgment unit judges that the handover time falls
below the present timer value.

14. A wireless terminal of a wireless communication system, the wireless
communication system including the wireless terminal and a plurality of
wireless base stations which communicate wirelessly with the wireless
terminal and performing control to lower the speed at which the wireless
terminal communicates with a communication party upon the time-out of a
timer value generated in accordance with the time required for
transmission/reception of a signal between the wireless terminal and a
communication party, the wireless base station requiring the present
timer value from the wireless terminal when performing control to lower
the speed at which the wireless station communicates with the wireless
terminal so as to increase the timer value in the case where the handover
execution is decided, the wireless terminal comprising:timer-value
calculation means which calculates the present timer value;a
timer-value-requirement receiver which receives the requirement from the
wireless base station; andtimer-value notification means which notifies
the wireless base station of the present timer value calculated by the
timer-value calculation means when the requirement is received by the
timer-value-requirement receiver.

15. A wireless terminal of a wireless communication system, the wireless
communication system including the wireless terminal and a plurality of
wireless base stations which communicate wirelessly with the wireless
terminal and performing control to lower the speed at which the wireless
terminal communicates with a communication party upon the time-out of a
timer value generated in accordance with the time required for
transmission/reception of a signal between the wireless terminal and a
communication party, the wireless base station enabling control to lower
the communication speed at which the wireless station communicates with
the wireless terminal so as to increase the timer value by receiving a
handover execution request, the wireless terminal
comprising:handover-execution judgment means which judges whether the
handover will be executed or not on the basis of a signal received from
the wireless base station;handover-execution-time decision means which
decides the execution time of the handover when the handover-execution
judgment means judges that the handover will be
executed;handover-execution-request transmission means which transmits
the handover execution request to the wireless base station when the
handover-execution judgment means judges that the handover will be
executed;enabling-acknowledgement reception means which receives an
enabling acknowledgement with respect to the handover execution request
from the wireless base station; andhandover-execution control means which
postpones the handover execution until after the execution time decided
by the handover-execution-time decision means when the enabling
acknowledgement is not received by the enabling-acknowledgement reception
means within a predetermined period of time after the transmission of the
handover execution request by the handover-execution-request transmission
means.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001]This application is a continuation Application of a PCT
international application No. PCT/JP2006/326215 filed on Dec. 28, 2006 in
Japan, the entire contents of which are incorporated by reference.

FIELD

[0002]The present invention relates to a handover method of a wireless
communication system, a wireless base station, and a wireless terminal of
the same system. For example, it relates to a technology preferably used
for a mobile communication system that includes a wireless terminal (MS:
Mobile Station) that can establish a mobile communication and a plurality
of wireless base stations (BS: Base Station) that communicate with the MS
and has a function of handover (HO) to switch BSs to be communicated with
as the MS moves.

BACKGROUND

[0003]In a wireless communication system including wireless terminals
(MSs) and wireless base stations (BSs) that provide wireless
communication zones, each MS can communicate with another MS via one of
those BSs if the MS is in a range (namely, service area) where the MS can
communicate wirelessly with that BS.

[0004]In such a wireless communication system, the communication breaks
down generally when an MS moves out of the area where the radio wave of
the communicating BS can reach (namely, out of the service area of that
BS). Therefore, the wireless communication system generally has handover
(HO) function, with which the communicating BS is switched to another as
the MS moves.

[0005]During the above-mentioned handover process, the link-layer
communication between the MS and the BS momentarily breaks down, and
therefore, the MS can not transmit packets to the communication-party MS
temporarily. Such packets that can not be transmitted to the
communication party are generally dropped in the course of their
transmission.

[0006]For example when an MS communicates with a communication party
according to TCP (Transmission Control Protocol), the MS measures time
(namely, RTT: Round Trip Time) between the transmission of a packet and
the reception of the ACK (ACKnowledgement) with respect to that packet
and calculates the time-out value (RTO: Retransmission TimeOut) of the
data-retransmission timer at the time of the retransmission control on
the basis of that RTT. When the ACK is not received in a communicating
process even after the timer exceeds the present RTO value (namely,
time-out), a congestion control is (for example, a control to lower the
communication speed considerably at which the MS communicates with a
communication party) exercised between the MS and a communication party.
Therefore, in such a case, the throughput may probably be lowered because
of slow-start algorithm after completion of the handover process.

[0007]In this regard, Non-Patent Documents 1 (IEEE Std 802.16e.-2005 and
IEEE Std 802.16.-2004/Cor 1-2005 (Amendment and Corrigendum to IEEE Std
802.16-2004)) and 2 (IEEE Std 802.16.-2004 (Revision of IEEE Std
802.16.-2001)) listed below disclose, for example, a method to inhibit
drop of the packets during a handover process by buffering the packets
transmitted from the communication party during the handover in the BS
and transmitting the packets to the MS after completion of the handover
process.

[0008]However, in the above-mentioned method, the ACKs with respect to the
packets transmitted from the MS are also buffered, and therefore, the
arrivals of the ACKs to the MS delay regularly. In such a case, there is
a high probability that the above-mentioned ACK response is received when
the timer exceeds the RTO value (namely, probability that the time-out of
the packet-retransmission timer occurs). In the result, the throughput is
lowered by the above-mentioned slow-start algorithm after completion of
the handover process.

[0009]In order to prevent such a lowering of throughput caused by arrival
delays of ACKs, the communication party may delay transmissions of ACKs
even in normal times so as to prolong the above-mentioned time to the
time-out sufficiently for executing the handover processes, for example.
However, in such a case, the MS is forced to be in an extra ACK-waiting
mode regularly, which leads to the throughput lowering even in normal
times.

SUMMARY

[0010]In a certain aspect, an object of the invention is to prevent the
lowering of throughput before and after handover processes in a wireless
communication system including a wireless terminal and wireless base
stations.

[0011]In a certain aspect, the following handover method of a wireless
communication system, wireless base station, and wireless terminal of the
same system.

[0012](1) A handover method of a wireless communication system, the
wireless communication system including a wireless terminal and a
plurality of wireless base stations which communicate wirelessly with the
wireless terminal and performing control to lower the speed at which the
wireless terminal communicates with a communication party upon the
time-out of a timer value generated in accordance with the time required
for transmission/reception of a signal between the wireless terminal and
a communication party, wherein the wireless base station performs the
control to lower the speed at which the wireless base station
communicates with the wireless terminal so as to increase the timer value
when the wireless base station decides execution of the handover of the
communication with the wireless terminal to another wireless base station
autonomously or in response to a request from the wireless terminal, and
subsequently, executes the handover.

[0013](2) In this case, it is possible that the wireless terminal
transmits a request for handover execution, and, when the wireless base
station receives the execution request, the wireless base station decides
the handover execution and transmits an enabling acknowledgement of the
handover to the wireless terminal.

[0014](3) In addition, it is possible that the wireless base station
judges whether the handover can be executed or not on the basis of
information about the execution condition of the handover, and, when
judging that it can be executed, decides the handover execution and
transmits an enabling acknowledgement of the handover to the wireless
terminal.

[0015](4) Moreover, it is possible that the wireless base station acquires
the present timer value from the wireless terminal, estimates the time
required for the handover, and performs control to lower the
communication speed on the condition that the present timer value is
smaller than the estimated time at the time of the handover-execution
decision.

[0016](5) In addition, it is possible that the wireless base station
records history information about the communication speed at the time of
generation of the timer value or acquisition thereof from the wireless
terminal, calculates the lowering value of the communication-speed
lowering control on the basis of the history information, and performs
control to lower the communication speed by the lowering value.

[0017](6) Moreover, it is possible that the wireless base station
reacquires the present timer value from the wireless terminal while
performing control to lower the communication speed, and performs control
to lower the communication speed until the estimated time falls below the
reacquired, present timer value.

[0018](7) Moreover, it is possible that the wireless terminal postpones
the handover when the wireless terminal does not receive the enabling
acknowledgement of the handover from the wireless base station with
respect to the request for handover execution within a predetermined
period of time.

[0019](8) In a certain aspect of the invention, a wireless base station of
a wireless communication system, the wireless communication system
including a wireless terminal and a plurality of the wireless base
stations which communicate wirelessly with the wireless terminal and
performing control to lower the speed at which the wireless terminal
communicates with a communication party upon the time-out of a timer
value generated in accordance with the time required for
transmission/reception of a signal between the terminal and a
communication party, the wireless base station including: handover
decision means which decides execution of the handover of the
communication with the wireless terminal to another wireless base station
autonomously or in response to a request from the wireless terminal;
communication-speed control means which increases the timer value by
performing control to lower the speed at which the wireless base station
communicates with the wireless terminal when the handover decision means
decides execution of the handover; and handover execution means which
executes the handover after the control to lower the communication speed
performed by the communication-speed control means.

[0020](9) In this case, it is possible that the handover decision means
includes a handover-execution-request receiver which receives the
handover execution request from the wireless terminal and decides
execution of the handover when the handover-execution-request receiver
receives the request for execution.

[0021](10) In addition, it is possible that the handover decision means
includes an execution-condition storage which stores information about
the execution condition of the handover and a handover judgment unit
which judges whether the handover can be executed or not on the basis of
the information stored in the execution-condition storage, and decides
execution of the handover when the handover judgment unit judges that the
handover can be executed.

[0022](11) Moreover, it is possible that the communication-speed control
means includes: a timer-value acquisition unit which acquires the present
timer value from the wireless terminal; a handover-time estimator which
estimates handover time, which is time required for the handover; and a
handover-time judgment unit which judges whether the present timer value
is smaller than the handover time estimated by the handover-time
estimator at the time of the handover-execution decision performed by the
handover decision means, and performs control to lower the communication
speed on the condition that the handover-time judgment unit judges that
the present timer value is smaller than the handover time estimated by
the handover-time estimator.

[0023](12) In addition, it is possible that the communication-speed
control means includes: a history record unit which records history
information about the communication speed at the time of generation of
the timer value or acquisition thereof from the wireless terminal; and a
lowering-value calculator which calculates the lowering value of the
communication-speed lowering control on the basis of the history
information recorded in the history record unit, and performs control to
lower the communication speed by the lowering value calculated by the
lowering-value calculator.

[0024](13) Moreover, it is possible that the communication-speed control
means includes a repeat controller which repeatedly performs, while
performing control to lower the communication speed, acquisition of the
present timer value by the timer-value acquisition unit, estimation by
the handover-time estimator, and judgment by the handover-time judgment
unit, until the handover-time judgment unit judges that the handover time
falls below the present timer value.

[0025](14) In a certain aspect of the invention, a wireless terminal of a
wireless communication system, the wireless communication system
including the wireless terminal and a plurality of wireless base stations
which communicate wirelessly with the wireless terminal and performing
control to lower the speed at which the wireless terminal communicates
with a communication party upon the time-out of a timer value generated
in accordance with the time required for transmission/reception of a
signal between the wireless terminal and a communication party, the
wireless base station requiring the present timer value from the wireless
terminal when performing control to lower the speed at which the wireless
terminal communicates with a communication party so as to increase the
timer value in the case where the handover execution is decided, the
wireless terminal including: timer-value calculation means which
calculates the present timer value; a timer-value-requirement receiver
which receives the requirement from the wireless base station; and
timer-value notification means which notifies the wireless base station
of the present timer value calculated by the timer-value calculation
means when the requirement is received by the timer-value-requirement
receiver.

[0026](15) In a certain aspect of the invention, a wireless terminal of a
wireless communication system, the wireless communication system
including the wireless terminal and a plurality of wireless base stations
which communicate wirelessly with the wireless terminal and performing
control to lower the speed at which the wireless terminal communicates
with a communication party upon the time-out of a timer value generated
in accordance with the time required for transmission/reception of a
signal between the wireless terminal and a communication party, the
wireless base station enabling control to lower the communication speed
at which the wireless terminal communicates with a communication party so
as to increase the timer value by receiving a handover execution request,
the wireless terminal including: handover-execution judgment means which
judges whether the handover will be executed or not on the basis of a
signal received from the wireless base station; handover-execution-time
decision means which decides the execution time of the handover when the
handover-execution judgment means judges that the handover will be
executed; handover-execution-request transmission means which transmits
the handover execution request to the wireless base station when the
handover-execution judgment means judges that the handover will be
executed; enabling-acknowledgement reception means which receives an
enabling acknowledgement with respect to the handover execution request
from the wireless base station; and handover-execution control means
which postpones the handover execution until after the execution time
decided by the handover-execution-time decision means when the enabling
acknowledgement is not received by the enabling-acknowledgement reception
means within a predetermined period of time after the transmission of the
handover execution request by the handover-execution-request transmission
means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
first embodiment of the present invention.

[0028]FIG. 2 is a sequence diagram illustrating an operation (handover
process) of the wireless communication system depicted in FIG. 1.

[0029]FIG. 3 is a sequence diagram illustrating an operation (handover
process) of the wireless communication system depicted in FIG. 1.

[0030]FIG. 4 illustrates an example of the format of the control message
according to the first embodiment.

[0031]FIG. 5 illustrates an example of each value of the control message
depicted in FIG. 4.

[0083]In the following, embodiments of the present invention will be
described with reference to drawings. However, it is to be understood
that the present invention is not limited to the embodiments and their
variations to be described below and various modifications can be added
thereto so long as they do not depart from the scope of the present
invention.

[A] Description of the First Embodiment

[0084]FIG. 1 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
first embodiment of the present invention. In the wireless communication
system 1 illustrated in FIG. 1, at least one wireless terminal (MS) 2 and
a plurality of wireless base stations (BS) 3 are connected to each other
via a wireless network (wireless link), and they can communicate
wirelessly with each other. In FIG. 1, the reference numeral 4 represents
one of the above-mentioned BSs 3 that is a handover-target wireless base
station (HO-target BS) of the MS 2. In addition, it is assumed that, in
the wireless communication system 1 of this example, the communication
speed (hereinafter, also referred to as "line speed") at which the MS 2
communicates with a communication party is controlled to be lowered upon
the time-out of RTO value, which is a timer value generated in accordance
with the time required for transmission/reception of a signal between the
MS 2 and a communication party. In this regard, the control (regulation)
of the communication speed is carried out by means of, for example,
specification (assignment) of number of channels used and symbol number
for the MS 2, as described later.

[0085]Moreover, in this example, not the BS 3 but the MS 2 can judge the
necessity of HO execution, and the HO-origin BS 3 can increase the RTO
value upon reception of an HO execution request from the MS 2 by
performing control to lower the speed of the communication with the MS 2
so as to simulate a line congestion of the wireless network.

[0088]The HO execution judgment unit 204 collects information about the
execution condition of an HO and judges whether the HO will be executed
or not. In the present example, it functions as the handover-execution
judgment means which judges whether an HO will be executed or not on the
basis of a signal received from the BS 3. In this context, the
above-mentioned information about the execution condition of an HO
includes, for example, information about the radio field strength of the
presently-communicating BS (this will be referred to as the "serving BS")
3. In this case, the HO execution judgment unit 204 measures the radio
field strength of the BS that is currently communicating with the MS 2
regularly or irregularly and decides an HO execution when it is judged
that the radio field strength is lower than the value preset by the
administrator. In addition, when the HO is executed, the HO execution
judgment unit 204 notifies the HO-target BS decider 206 of the HO
execution, as well as requests it to decide the HO-target BS 4.

[0089]The adjacent BS information registrar 253 registers (stores), in the
adjacent BS information storage 211, various kinds of information
(namely, adjacent BS information) (for example, BS identifier, frequency
used by the BS, radio field strength of the BS, capability types
supported, number of MS 2 processed currently, etc.) about adjacent, one
or more of other BSs 3, which is preset or collected through
communication with each corresponding BS 3. The adjacent BS information
registrar 253 is composed of a shared or dedicated memory, for example.

[0090]The adjacent BS information storage 211 stores various information
of BSs, which is registered by the above-mentioned adjacent BS
information registrar 253. FIG. 7 illustrates an example of the adjacent
BS information to be stored in the adjacent BS information storage 211.
In the example illustrated in FIG. 7, information of three BSs 3 (whose
BS identifiers are BS#02, BS#03, and BS#04) including the frequency used,
the radio field strength, and the BSs capabilities (capability types
supported including: security function; retransmission function; and MIMO
(Multiple Input Multiple Output), and the number of MSs in charge) is
specified for each BS 3 (for each BS identifier). Concerning the BS 3
whose BS identifier is BS#02, its frequency used is "F#02", radio field
strength is "HIGH", security is "SUPPORTED", retransmission function is
"SUPPORTED", MIMO function is "NOT SUPPORTED", and the number of MSs in
charge is "20".

[0091]The HO-target BS decider 206 decides the HO-target BS 4 by selecting
from the BSs registered in the adjacent BS information storage 211 with
the reception of the HO execution notification from the HO execution
judgment unit 204 as a trigger. For example, the HO-target BS decider 206
can select (decide) the BS whose "radio field strength" information in
the adjacent BS information illustrated in FIG. 7 is higher than those of
the other BSs 3 as the HO-target BS 4. In addition, the HO-target BS
decider 206 requests the HO-execution-notification transmitter 207 to
transmit an HO execution notification after the HO-target BS 4 is
decided.

[0092]The HO-execution-notification transmitter 207 transmits the HO
execution notification received from the HO-target BS decider 206 to the
BS 3 via the distributor 251. The HO-execution-notification transmitter
207 functions as the handover-execution-request transmission means which
transmits the handover execution request to the BS 3 when the
above-mentioned HO execution judgment unit 204 judges (decides) that the
handover will be executed. For that purpose, the
HO-execution-notification transmitter 207 has a function of, for example,
creating a control message used for the HO execution notification and
transmitting it to the distributor 251.

[0093]The RTO-request receiver (timer-value-request receiver) 201 receives
a request (RTO value request notification) for the present RTO value from
the BS 3. The RTO-request receiver 201 requests the RTO calculator 202 to
calculate the present RTO value with the reception of the RTO value
request notification as a trigger, and acquires the calculated value
stored in the RTO value storage 252, which is the newest (present) RTO
value, so as to transmit it to the RTO value transmitter 203 (namely, so
as to request the RTO value transmitter 203 to send it).

[0094]The RTO calculator (timer-value calculation means) 202 measures the
time (RTT: Round Trip Time) between the transmission of the upload data
(TCP data) to a communication party via the serving BS 3, and the return
of the ACK, with the RTO calculation request from the RTO-requirement
receiver 201 as a trigger, and calculates the present RTO value on the
basis of the measured RTT. The result of the calculation (namely, RTO
value) is stored in the RTO storage 252 at all times. Specifically, in
the present example, the RTO value is calculated from the RTT with the
following formulae (1) to (3), for example.

SRTT=7/8SRTT+1/8RTT (1)

v=v+1/8(|RTT-SRTT|-v) (2)

RTO=SRTT+4×v (3)

[0095]In the above formulae, the left-hand side of each formula (1) to (3)
represents the renewed value, SRTT represents the smoothed RTT, and v
represents the average deviation of RTT. In addition, the RTO calculator
202 also has a function of performing retransmission control on the basis
of the calculated RTO value.

[0096]The RTO value storage 252 stores the RTO value calculated by the RTO
calculator 202, as described above. The RTO value storage 252 is composed
of a shared or dedicated memory, for example.

[0097]The RTO value transmitter 203 transmits the present RTO value that
is transmitted from the RTO-request receiver 201 to the BS 3. In the
present example, the RTO value transmitter 203 creates a control message
containing the RTO value and transmits the control message to the
distributor 251, with the RTO-value notification from the RTO-request
receiver 201 as a trigger. Namely, the RTO value transmitter 203
functions as the timer-value notification means which notifies the BS 3
of the present RTO value calculated by the RTO calculator 202 when the
RTO-value request is received by the RTO-request receiver 201.

[0098]The HO-execution-confirmation receiver (enabling-acknowledgement
reception means) 208 receives, from the BS 3, an
HO-execution-confirmation notification (enabling acknowledgement) with
respect to the HO execution notification (execution request) sent from
the HO-execution-notification transmitter 207. In the present example,
the HO-execution-confirmation receiver 208 notifies the HO processor 210
of an HO execution, with the reception of the above-mentioned
HO-execution-confirmation notification as a trigger.

[0099]The HO processor 210 executes an HO process, with which the
communication process is handed over from the serving BS 3 to the
HO-target BS 4, with the reception of the HO execution notification from
the HO-execution-confirmation receiver 208 as a trigger.

[0100]The distributor 251 controls the connection identifiers (CID:
Connection IDentifier), which represent the communication paths to the
serving BS 3, and outputs control messages to appropriate targets in
accordance with the information (TYPE value) about the message types
(such as HO execution notification, HO-execution-confirmation
notification, RTO request (acquisition), RTO notification (transmission),
and line speed regulation to be described later) contained in the header
of each control message. In the present example, a format illustrated in
FIG. 4 is used for the control message, for example.

[0101]The format of the control message illustrated in FIG. 4 is composed
of a header portion and a data portion. In the header portion, the
above-mentioned CID and TYPE values are contained. In the data portion,
contents (namely, DATA values) of the data (message) corresponding to
each TYPE value (namely, message type) is contained. In FIG. 5, examples
of the CID, the TYPE value, and the DATA value are illustrated.

[0102]As illustrated in FIG. 5, the TYPE values which are corresponding to
each message type are defined for each communication path (CID).
Therefore, the distributor 251 can identify the communication path and
the message type by referring to the CID and the TYPE value contained in
the header portion of the received control message.

[0103]For example, in a control message expressing an "HO execution
notification" addressed to the BS 3, "TYPE#00" is designated as the
message type (TYPE value), and the identifier (MS#01, in the example of
FIG. 5) of the MS that is the HO-request origin, and the identifier
(BS#02, in the example of FIG. 5) of the BS 4, which is the HO-target,
are designated as the message content (DATA value). In a control message
expressing an "HO execution confirmation" addressed to the MS 2, which is
a reply to the above message, "TYPE#02" is designated as the message type
(TYPE value), and information indicating enabling (OK) or disabling (NG)
of HO is designated as the message content (DATA value).

[0104]In a control message expressing an "RTO request (acquisition)"
addressed to the MS 2, "TYPE#03" is designated as the message type (TYPE
value) (and no particular value is designated as the message content
(DATA value)). In a control message expressing an "RTO transmission"
addressed to the MS 3[sic], which is a reply to the above message,
"TYPE#04" is designated as the message type (TYPE value), and the present
RTO value is designated as the message content (DATA value).

[0105]In the same way, in a control message expressing a "line speed
regulation" addressed to the MS 2, "TYPE#01" is designated as the message
type (TYPE value), and the number of channels used and the symbol number
(both are "1" in the example of FIG. 5) are designated as the message
content (DATA value)).

[0106]With the above-mentioned structure, the MS 2 according to the first
embodiment of the present invention can decide HO execution and request
the BS 3 to perform HO upon reception of a signal from the BS 3, as well
as can transmit the RTO value which is calculated by the RTO calculator
202 to the BS 3 in accordance with a request from the BS 3. In addition,
the MS 2 can perform congestion control (communication speed control) to
lower the line speed of the communication with the communication party
upon time-out of the RTO value.

[0109]The HO-execution-notification receiver 101 receives HO execution
notifications transmitted from the MS 2. In the present example, the
HO-execution-notification receiver 101 notifies the HO time estimator 107
of information (BS identifier: refer to FIG. 5) about the HO-target BS 4
which is contained in the HO execution notification so as to request the
HO time estimator 107 to estimate the HO processing time, and also
requests the RTO requirement unit 108[sic] to acquire the RTO value, for
example.

[0110]Namely, the HO-execution-notification receiver 101 performs the
function of the handover-execution-request receiver which receives the HO
execution request from the MS 2, as well as performs one of the functions
of the handover decision means which decides execution of HO of the
communication with the MS 2 to another BS 3 (HO-target BS 4).

[0111]The HO time estimator (handover-time estimator) 107 estimates time
required for the HO process from the serving BS 3 to the HO-target BS 4
in accordance with a predetermined HO procedure, with the reception of
the estimation request from the HO-execution-notification receiver 101 as
a trigger. Specifically, the HO time estimator 107 estimates HO
processing time according to, for example, time required for negotiation
between the MS 2 and the HO-target BS 4 during the function-by-function
HO process, which is preset by the administrator, after acquiring
information about the HO-target BS 4 from the adjacent BS information
storage 111 on the basis of the BS identifier of the HO-target BS 4,
which was sent from the HO-execution-notification receiver 101, and
information about the MS 2, which is the communication-origin, from the
MS information storage 152. The HO time estimator 107 can notify the
line-speed-regulation judgment unit 108 of the estimated HO processing
time.

[0112]Namely, the HO time estimator 107 estimates the HO processing time
with, for example, a table of times required for negotiation processes
specified for each function, such as illustrated in FIG. 8, which is
stored in the MS information storage 152. In the example illustrated in
FIG. 8, the above-mentioned HO processing time can be estimated so that
20 ms+10 ms=30 ms, in the case where the MS 2 and the BS 3 support the
security function and the MIMO function.

[0113]The adjacent BS information registrar 153 registers various
information about one or more of the BSs 3 with the adjacent BS
information storage 111, in the same way as the adjacent BS information
registrar 253 of the MS 2 side. For example, the adjacent BS information
registrar 153 registers (stores) information (adjacent BS information)
about one or more of the BSs 3 that are geographically neighboring on the
serving BS 3 in the adjacent BS information storage 211[sic] in advance
or by acquiring it through communication. The adjacent BS information
includes, for example, the BS identifier, the frequency used by the BS,
the radio field strength of the BS, the capability types supported, the
number of MS processed currently and the like, as described earlier.

[0114]The adjacent BS information storage 111 stores the adjacent BS
information registered by the adjacent BS information registrar 153, and
thereby, the same adjacent BS information is shared with the MS 2
(adjacent BS information storage 211). The adjacent BS information
storage 111 is also composed of a shared or a dedicated memory.

[0115]The MS information storage 152 contains (stores) information about
the MSs 2 that are communicating with the BS 3. The MS information
storage 152 can store, for example, MS information, which is specified
for each MS identifier, including: the aforementioned CID; the channel
capacity (symbol number and channel number) of the communication path,
which can be distinguished by the CID; and the functions supported by the
corresponding MS 2 (namely, MS capabilities), as illustrated in FIG. 6.

[0117]The RTO request unit (timer-value acquisition unit) 106 requests the
MS 2 to send the RTO value and acquires the present RTO value from the MS
2, with the reception of the RTO-value acquisition request from the
HO-execution-notification receiver 101 as a trigger. The RTO request unit
106 can request the MS 2 to send RTO by creating a control message (refer
to FIG. 5) expressing an "RTO request" and transmitting the control
message to the distributor 151. Also, the RTO request unit 106 can
acquire the RTO value from the MS 2 by receiving a control message (refer
to FIG. 5) expressing an "RTO transmission", which is the reply to the
above-mentioned transmitted control message, from the MS 2.

[0118]The line-speed-regulation judgment unit 108 judges whether an HO can
be executed or not on the basis of the information (information about the
execution condition) stored in the adjacent BS information storage 111
and the MS information storage 152 and judges whether an HO can be
executed or not also on the basis of the RTO value sent by the RTO
requirement unit 106 and the HO processing time estimated by the HO time
estimator 107. When the HO can be executed, the line-speed-regulation
judgment unit 108 decides the HO execution and judges whether the line
speed will be regulated or not.

[0119]Specifically, the line-speed-regulation judgment unit 108 requests
the line speed regulator 102 to perform a lowering control of the speed
of the communication with the MS 2 when it is judged that the
above-mentioned present RTO value is smaller than the HO time estimated
by the above-mentioned HO time estimator 107. On the other hand, when the
present RTO value is larger than the estimated HO time, the
line-speed-regulation judgment unit 108 does not request the lowering
control but requests the HO-execution-confirmation notifier 103 to
transmit an HO-execution-confirmation notification.

[0120]Namely, the line-speed-regulation judgment unit 108 of the present
example performs functions both of the handover judgment unit which
judges whether an HO can be executed or not on the basis of the
information about the above-mentioned execution condition which is stored
in the above-mentioned execution-condition storage 111, 152 and of the
handover-time judgment unit which judges whether the above-mentioned
present RTO value is smaller than the handover time estimated by the
above-mentioned HO time estimator 107 at the time of the HO-execution
decision which is made when the HO can be executed.

[0121]The line speed regulator 102 regulates the speed of the
communication with the MS 2 via the wireless network. The line speed
regulator 102 performs control to lower the speed of the communication
with the MS 2 via the wireless network to a preset value, with the
reception of the speed-lowering-control request from the above-mentioned
line-speed-regulation judgment unit 108 as a trigger. Namely, the line
speed regulator 102 creates a control message (refer to FIG. 5)
expressing a "line speed regulation" and transmits the control message to
the MS 2 via the distributor 151. When the control of communication speed
is completed, the line speed regulator 102 requests the
HO-execution-confirmation notifier 103 to transmit an
HO-execution-confirmation notification.

[0122]Concerning the speed control of the communication with the MS 2, it
can be performed by, for example, assigning the frequency range (namely,
the channel number) and the time for which the range can be occupied
(namely, the symbol number) to the MS 2 using a control message.
Specifically, it can be performed by transmitting a control message to
the target MS 2, in which the CID acquired from the MS information
storage 152 and the message type are specified at the header portion and
the channel number and the symbol number are specified at the data
portion, as described earlier for FIG. 5. Incidentally, the line speed
regulator 102 of the HO-target BS 4 can assign an arbitrary speed of the
communication with the MS 2 using a similar control message, and
therefore, it can perform control to increase the communication speed,
which was lowered before the execution of the HO.

[0123]From the above description, it is evident that the above-mentioned
RTO request unit 106, HO time estimator 107, line-speed-regulation
judgment unit 108, and line speed regulator 102 function in cooperation
as the communication-speed control means which increases the RTO value by
performing control to lower the speed of the communication with the MS 2
when the above-mentioned HO-execution-notification receiver 101 decides
the above-mentioned HO execution, and that they can perform control to
increase the above-mentioned communication speed after completion of the
HO execution.

[0124]The HO-execution-confirmation notifier 103 transmits an
HO-execution-confirmation notification (enabling acknowledgement), which
expresses an HO-execution enabling, to the MS 2 using a control message
(refer to FIG. 5), with the notification (request) sent from the line
speed regulator 102 as a trigger. In the present example, the
HO-execution-confirmation notifier 103 functions as the handover
execution means which executes an HO process after the control to lower
the communication speed, which is performed by the line speed regulator
102. Specifically, the HO-execution-confirmation notifier 103 creates a
control message in which the CID acquired from the MS information storage
152 and the message type are specified at the header portion and the
HO-execution enabling (OK) information is specified at the data portion,
as described earlier for FIG. 5, with the notification sent from the line
speed regulator 102 as a trigger, and transmits it to the distributor
151.

[0125]The distributor 151 controls the CIDs with respect to the
communicating MSs 2 and outputs control messages to appropriate targets
in accordance with the information (TYPE value) about the message types
(such as HO execution notification, HO-execution-confirmation
notification, RTO request (acquisition), RTO notification (transmission),
and line speed regulation to be described later) contained in the header
of each control message.

[0126]With the above-mentioned structure, the BS 3 according to the first
embodiment of the present invention estimates time required for an HO
process upon receiving an HO execution notification from the MS 2,
compares the estimated result and the present RTO value which was
acquired by sending a request to the MS 2, regulates (performs control to
lower) the line speed on the basis of the comparison result, and then
executes the HO. In this way, the BS 3 can execute an HO process with the
RTO value increased in order for the timer not to exceed the RTO value by
creating simulated line congestion through a control to lower the speed
of the communication with the MS 2, if it is necessary.

[0127](Description of Entire Operation of Wireless Communication System 1)

[0128]Next, an example of the entire operation of the wireless
communication system 1 having the above-mentioned structure will be
described with reference to FIG. 2 and FIG. 3. FIG. 2 is a sequence
diagram illustrating an operation (handover process) of the wireless
communication system (especially, the MS 2) illustrated in FIG. 1. FIG. 3
is a sequence diagram illustrating an operation (handover process) of the
wireless communication system (especially, the BS 3) illustrated in FIG.
1.

[0129]First, the MS 2 acquires the CID (for example, CID#01) of the
connection to the serving BS 3 in advance. The CID (CID#01) is controlled
by the distributor 251.

[0130]In addition, in the MS 2, the adjacent BS information registrar 253
acquires information (adjacent BS information) about the BSs 3 (including
the HO-target BS 4) that are geographically neighboring on the serving BS
3, such as their frequencies used, capability types supported (typically
including security function, packet retransmission function, MIMO
function, etc.), radio field strengths, and the number of MSs in charge,
and registers it in the adjacent BS information storage 211 (step S1).
The MS 2 can acquire that adjacent BS information by, for example,
receiving it when it is transmitted (reported) from the BS 3.

[0131]Moreover, in the MS 2, the RTO calculator 202 measures the RTTs of
the TCP data that are transmitted/received to/from the communication
parties (namely, other MSs and Bss 3) regularly or irregularly and
calculates the RTO values from those RTTs so as to store them in the RTO
value storage 252 (step S2).

[0132]On the other hand, the BS 3 acquires information (MS information)
about the MS 2 such as MS 2 identifier (for example, MS#01) and
capability types supported by the MS 2 (typically including security
function, packet retransmission function, MIMO function, etc.), and
registers it in the MS information storage 152. That MS information can
be acquired by, for example, transmission of necessary information also
from the MS 2 at the phase when the MS 2 starts to communicate with the
BS 3.

[0133]Next, in the BS 3, the adjacent BS information registrar 153
acquires information (adjacent BS information) about the geographically
neighboring BSs 3 (including the HO-target BS 4) in advance, such as the
frequency used and the capability types supported, and registers it in
the adjacent BS information storage 111 (step S3). That adjacent BS
information can be acquired by, for example, exchanging information with
the neighboring BSs 3 (including the HO-target BS 4) regularly or
irregularly with the above-mentioned CIDs to be used for exchanging BS
information.

[0134]After the above-mentioned preliminary processes such as registration
of information in the MS 2 and the BS 3, the MS 2 checks whether an HO
should be executed or not by measuring the radio field strength of the
serving BS 3 regularly or irregularly by the HO execution judgment unit
204. When the radio field strength is lower than the value that was
preset by the administrator with a command, the HO execution judgment
unit 204 judges that an HO execution is necessary and requests the
HO-target BS decider 206 to decide the HO-target BS 4 (step S4).

[0135]The HO-target BS decider 206 refers to the adjacent BS information
storage 211, with the above-mentioned request from the HO execution
judgment unit 204 as a trigger, and determines the HO-target BS 4 on the
basis of the adjacent BS information (step S5). For example, the BS 3 of
which radio field strength is the highest of the neighboring BSs 3 is
determined to be the HO-target BS 4. Then, the HO-target BS decider 206
notifies the HO-execution-notification transmitter 207 of the BS
identifier (for example, BS#02) of the determined HO-target BS 4, and
also, requests the HO-execution-notification transmitter 207 to transmit
an HO execution notification (step S6).

[0136]The HO-execution-notification transmitter 207 creates a control
message, upon reception of the above-mentioned request from the HO-target
BS decider 206, in which the message type (TYPE#00) expressing an "HO
execution notification" is specified at the header portion and the BS
identifier (BS#02) of the HO-target BS 4 and the MS identifier (MS#01) of
the MS 2, of which communication process is going to be handed over, are
specified at the data portion and transmits the control message to the
distributor 251 (step S7).

[0137]The distributor 251 specifies the CID (CID#01) at the header portion
of that control message and transmits that control message to the BS 3
via the wireless network (step S8).

[0138]In the BS 3, the HO-execution-notification receiver 101 receives
that control message (HO execution notification) via the distributor 151,
which is transmitted from the MS 2 (step S9), acquires the MS identifier
(MS#01) and the BS identifier (BS#02) contained in the data portion of
that HO execution notification, and then sends a request for estimation
of the HO processing time to the HO time estimator 107 together with the
BS identifier (step S10). In addition, the HO-execution-notification
receiver 101 acquires the MS identifier (MS#01) from the data portion of
that control message and notifies the HO time estimator 107 of the MS
identifier (MS#01).

[0139]The HO time estimator 107 acquires the MS information, from the MS
information storage 152, about the MS 2 that transmitted the HO execution
notification on the basis of the above-mentioned MS identifier (MS#01)
(step S11), and transmits information about the functions supported by
the MS 2 to the BS 3 (HO-target BS 4) of which identifier is the
above-mentioned BS identifier (BS#02). In addition, the HO time estimator
107 estimates time (HO processing time) required for negotiation between
the MS 2 and the BS 3 at the time of HO process on the basis of the
information about the negotiation which is stored in the adjacent BS
information storage 111, and transmits the estimated HO processing time
to the line speed regulator 108. Moreover, the HO time estimator 107
notifies the RTO request unit 106 of an RTO acquisition request in order
to acquire the RTO value of the MS 2 corresponding to the MS identifier
that is acquired from the HO-execution-notification receiver 101 (step
S12).

[0140]The RTO request unit 106 creates a control message expressing "RTO
acquisition" when the RTO request unit 106 receives the above-mentioned
RTO-value acquisition request from the HO time estimator 107. For
example, the RTO request unit 106 searches the MS information storage 152
using the above-mentioned MS identifier (MS#01) as a search key so as to
acquire the CID (CID#01) of the corresponding MS 2 (step S13), and
specifies that CID at the header portion of the control message, as well
as specifies a message type (TYPE#03) expressing "RTO acquisition" at the
header portion. Then the RTO request unit 106 transmits the control
message to the distributor 151 (step S14).

[0141]Upon receiving it, the distributor 151 transmits that control
message to the MS 2 via the wireless network (step S15).

[0142]In the MS 2, the RTO-request receiver 201 receives the
above-mentioned control message expressing "RTO acquisition" from the BS
3 via the distributor 251 (step S16), acquires the present RTO value from
the RTO value storage 252 (step S17), and requests the RTO value
transmitter 203 to transmit the RTO value while notifying it of the RTO
value (step S18).

[0143]The RTO value transmitter 203 creates a control message by
specifying the RTO value sent from the RTO-requirement receiver 201 at
the data portion and specifying a message type (TYPE#04) expressing "RTO
transmission" at the header portion, and transmits that control message
to the distributor 251 (step S19).

[0144]Upon receiving it, the distributor 251 adds the CID (CID#01) at the
header portion of that control message and transmits that control message
to the BS 3 via the wireless network (step S20).

[0145]Next, in the BS 3, the RTO request unit 106 receives the
above-mentioned control message expressing "RTO transmission" (step S21),
acquires the RTO value specified at the data portion of the message, and
transmits the acquired RTO value to the line-speed-regulation judgment
unit 108 (step S22).

[0146]The line-speed-regulation judgment unit 108 compares the RTO value
acquired from the RTO request unit 106 with the HO processing time
estimated by the HO time estimator 107 (step S23). When the RTO value is
larger than the HO processing time (see Y route of step S23), the
line-speed-regulation judgment unit 108 judges that the line speed
regulation is not necessary and requests the HO-execution-confirmation
notifier 103 to transmit an HO-execution-confirmation notification by
notifying it of the above-mentioned MS identifier (MS#01) (step S28).

[0147]On the other hand, when the RTO value is equal to or smaller than
the HO processing time (see N route of step S23), the
line-speed-regulation judgment unit 108 judges that the line speed
regulation (lowering control) is necessary, because the timer will exceed
the RTO value during the HO process without it, and requests the line
speed regulator 102 to regulate (perform control to lower) the line speed
by notifying it of the above-mentioned MS identifier (MS#01).

[0148]Upon the reception of the above-mentioned request for line speed
regulation, the line speed regulator 102 creates a control message in
which, as described earlier for FIG. 5, the symbol number and channel
number (both of which are "1" in the present example, for example) are
specified at the data portion and a message type (TYPE#01) expressing
"line speed regulation" and the CID (CID#01) of the corresponding MS 2,
which is acquired from the MS information storage 152 using the
above-mentioned MS identifier (MS#01) as the search key, are specified at
the header portion, and transmits that control message to the distributor
151 (steps S24, S25).

[0149]Upon receiving it, the distributor 151 transmits that control
message to the MS 2 via the wireless network (step S26).

[0150]Subsequently, the line speed regulator 102 requests the
HO-execution-confirmation notifier 103 to transmit an
HO-execution-confirmation notification by notifying it of the
above-mentioned MS identifier (MS#01) (step S27).

[0151]Upon the reception of the above-mentioned request for transmission
of the HO-execution-confirmation notification from the
line-speed-regulation judgment unit 108 or the line speed regulator 102,
the HO-execution-confirmation notifier 103 creates a control message in
which, as described earlier for FIG. 5, the information expressing "HO
execution enabling" is specified at the data portion and a message type
(TYPE#02) expressing "HO execution confirmation" and the CID (CID#01) of
the corresponding MS 2, which is acquired from the MS information storage
152, are specified at the header portion, and transmits that control
message to the distributor 151 (steps S29, S30).

[0152]Upon receiving it, the distributor 151 transmits that control
message to the MS 2 via the wireless network (step S31).

[0154]Incidentally, the line speed, which was controlled to be lowered by
the aforementioned procedure before executing the HO, is reassigned to
the MS 2 by the HO-target BS 4 independently of the state before the HO
execution after completion of the HO. Concerning the reassignment method
and the line speed, it is possible that, for example, the HO-target BS 4
determines the line speed to be reassigned to the MS 2 in accordance with
the number of the currently-processed MSs and notifies the MS 2 of the
determined line speed using a control message (TYPE#01) when the MS 2
communicates with the HO-target BS 4.

[0155]As described above, according to the first embodiment of the present
invention, when an HO execution is decided by the MS 2 and the
requirement for it is sent to the BS 3, the BS 3 performs control to
lower the line speed of the communication with the MS 2 before executing
the HO, which causes a simulated congestion of the line. This can
increase the RTO value to the degree that the RTO value is longer than
the time that is necessary for completion of the HO process. Therefore,
the time-out of the RTO value (retransmission timer) during the HO can be
prevented, which makes it possible to inhibit the throughput reduction
caused by a slow-start algorithm originating from the above-mentioned
time-out without inducing throughput reduction in normal times.

[A1] Description of the Variation of the First Embodiment

[0156]In the above-mentioned first embodiment, explanation was given on
the case where the necessities of HO processes are checked and decided by
the MS 2, in other words, the MS 2 takes the initiative to execute HO
processes. However, the BS 3 may take the initiative to execute HO
processes. Such an embodiment will be described in the explanation of the
present variation example with reference to FIG. 9 to FIG. 11.

[0157]FIG. 9 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
variation of the above-mentioned first embodiment. FIG. 10 is a sequence
diagram illustrating an operation (handover process) of the wireless
communication system (especially, the MS 2a) illustrated in FIG. 9. FIG.
11 is a sequence diagram illustrating an operation (handover process) of
the wireless communication system (especially, the BS 3a) illustrated in
FIG. 9.

[0158]Namely, in the wireless communication system 1a illustrated in FIG.
9, at least one MS 2a and a plurality of BSs 3a (including the HO-target
BS 4a) are connected to each other via a wireless network, in the same
way as the wireless communication system 1 of the first embodiment.
However, it differs from the wireless communication system 1 of the first
embodiment in that the BS 3a decides whether HO processes will be
executed or not.

[0159](Description of MS 2a)

[0160]Therefore, the MS 2a of the present example comprises an adjacent BS
information storage 211a, an adjacent BS information registrar 253a, an
RTO-request receiver 201a, an RTO calculator 202a, an RTO value storage
252a, an RTO value transmitter 203a, an HO-execution-confirmation
receiver 208a, an HO processor 210a, and a distributor 251a, which are of
the same or similar functions as or to each corresponding units 211, 253,
201, 202, 252, 203, 208, 210 and 251 described earlier for FIG. 1 to FIG.
3, and it may not need the functions of the HO execution judgment unit
204 and the HO-target BS decider 206, which were illustrated in FIG. 1.

[0161](Description of BS 3a)

[0162]On the other hand, the BS 3a of the present example comprises an RTO
request unit 106a, an HO time estimator 107a, a line-speed-regulation
judgment unit 108a, an adjacent BS information registrar 153a, an
adjacent BS information storage 111a, an MS information storage 152a, a
line speed regulator 102a, an HO-execution-confirmation notifier 103a,
and a distributor 151a, which are of the same or similar functions as or
to each corresponding units described earlier for FIG. 1, and also
comprises an HO execution judgment unit 104a and an HO-target BS decider
105a, in place of the above-mentioned HO-execution-notification receiver
101 described earlier for FIG. 1.

[0163]The HO execution judgment unit 104a of the BS 3a collects
information about the execution condition of HO processes and judges
whether the HO processes will be executed or not. When the HO is
executed, the HO execution judgment unit 104a notifies the HO-target BS
decider 105a of the HO execution and requests the HO-target BS decider
105a to decide the HO-target BS 4a. Namely, the HO execution judgment
unit 104a performs one of the functions of the handover decision means
which autonomously decides execution of HO of the communication with the
MS 2a to another BS 3a (HO-target BS 4a).

[0164]The HO-target BS decider 105a decides the HO-target BS 4a on the
basis of the adjacent BS information stored in the adjacent BS
information storage 111a by selecting from the adjacent BSs 3a that are
already registered, with the reception of the request for decision of the
HO-target BS 4a from the HO execution judgment unit 104a as a trigger.
Concerning the criterion of deciding (selecting) the BS, the BS 3a whose
radio field strength is higher than those of the other BSs 3a can be
selected (decided) as the HO-target BS 4a, for example, also in the
present variation.

[0165](Description of Entire Operation of Wireless Communication System
1a)

[0166]In the following, an example of the entire operation of the wireless
communication system 1a of which construction is such as described above
is illustrated in FIG. 10 and FIG. 11.

[0167]First, the MS 2a acquires the CID (for example, CID#01) of the
connection to the serving BS 3a in advance. The CID (CID#01) is
controlled by the distributor 251a.

[0168]In addition, in the MS 2a, the adjacent BS information registrar
253a acquires adjacent BS information about the BSs 3a (including the
HO-target BS 4a) that are geographically neighboring on the serving BS
3a, and registers it in the adjacent BS information storage 211a (step
S1a), as illustrated in FIG. 10. Moreover, in the MS 2a, the RTO
calculator 202a measures the RTTs of the TCP data that are
transmitted/received to/from the communication parties (namely, other MSs
and Bss 3a) regularly or irregularly and calculates the RTO values from
those RTTs so as to store them in the RTO value storage 252a (step S2a).

[0169]On the other hand, the BS 3a acquires information (MS information)
about the MS 2a, such as the identifier of the MS 2a (for example, MS#01)
and capability types supported by the MS 2a (typically including security
function, packet retransmission function, MIMO function, etc.), and
registers it in the MS information storage 152a. That MS information can
be acquired by, for example, transmission of necessary information also
from the MS 2a at the phase when the MS 2a starts to communicate with the
BS 3a.

[0170]Next, in the BS 3a, the adjacent BS information registrar 153a
acquires the adjacent BS information about the geographically neighboring
BSs 3a (including the HO-target BS 4a) in advance, and registers it in
the adjacent BS information storage 111a (step S3a). That adjacent BS
information can be acquired by, for example, exchanging information with
the neighboring BSs 3a (including the HO-target BS 4a) regularly or
irregularly with the above-mentioned CIDs to be used for exchanging BS
information.

[0171]After the above-mentioned preliminary processes such as registration
of information in the MS 2a and the BS 3a, the BS 3a checks whether an HO
should be executed or not by measuring the radio field strength of the MS
2a regularly or irregularly by the HO execution judgment unit 104a. When
the radio field strength is lower than the value that was preset by the
administrator with a command, the HO execution judgment unit 104a judges
that an HO execution is necessary and requests the HO-target BS decider
106a[sic] to decide the HO-target BS 4a (step S4a).

[0172]The HO-target BS decider unit 106a[sic] refers to the adjacent BS
information storage 111a, with the above-mentioned request from the HO
execution judgment unit 104a as a trigger, and determines the HO-target
BS 4a on the basis of the adjacent BS information (step S5a). For
example, the BS 3a of which radio field strength is the highest of the
neighboring BSs 3a is determined to be the HO-target BS 4a. Then, the
HO-target BS decider 106a[sic] sends a request for estimation of the HO
processing time to the HO time estimator 107a together with the BS
identifier (BS#02) of the HO-target BS 4a (step S10a). In addition, the
HO-target BS decider 106a[sic] notifies the HO time estimator 107a of the
MS identifier (MS#01) corresponding to the MS 2a of which communication
is going to be handed over.

[0173]The HO time estimator 107a acquires the MS information, from the MS
information storage 152a, about the corresponding MS 2a on the basis of
the above-mentioned MS identifier (MS#01) (step S11a), and transmits
information about the functions supported by the MS 2a to the BS 3a
(HO-target BS 4a) that possesses the above-mentioned BS identifier
(BS#02). In addition, the HO time estimator 107a estimates time (HO
processing time) required for negotiation between the MS 2a and the BS 3a
at the time of HO process on the basis of the information about the
negotiation which is stored in the adjacent BS information storage 111a,
and transmits the estimated HO processing time to the line speed
regulator 108a.

[0174]The RTO request unit 106a creates a control message expressing "RTO
acquisition" when the RTO request unit 106a receives the above-mentioned
RTO-value acquisition request from the HO time estimator 107a (step
S12a). For example, the RTO request unit 106a searches the MS information
storage 152a using the above-mentioned MS identifier (MS#01) as a search
key so as to acquire the CID (CID#01) of the corresponding MS 2a (step
S13a), and specifies that CID at the header portion of the control
message, as well as specifies the message type (TYPE#03) expressing "RTO
acquisition" at the header portion. Then the RTO request unit 106a
transmits the control message to the distributor 151a (step S14a).

[0175]Upon receiving it, the distributor 151a transmits that control
message to the MS 2a via the wireless network (step S15a).

[0176]In the MS 2a, the RTO-request receiver 201a receives the
above-mentioned control message expressing "RTO acquisition" from the BS
3a via the distributor 251a (step S16a), acquires the present RTO value
from the RTO value storage 252a (step S17a), and requests the RTO value
transmitter 203a to transmit the RTO value while notifying it of the RTO
value (step S18a).

[0177]The RTO value transmitter 203a creates a control message by
specifying the RTO value sent from the RTO-request receiver 201a at the
data portion and specifying a message type (TYPE#04) expressing "RTO
transmission" at the header portion, and transmits that control message
to the distributor 251a (step S19a).

[0178]Upon receiving it, the distributor 251a adds the CID (CID#01) at the
header portion of that control message and transmits that control message
to the BS 3a via the wireless network (step S20a).

[0179]Next, in the BS 3a, the RTO request unit 106a receives the
above-mentioned control message expressing "RTO transmission" (step
S21a), acquires the RTO value specified at the data portion of the
message, and transmits the acquired RTO value to the
line-speed-regulation judgment unit 108a (step S22a).

[0180]The line-speed-regulation judgment unit 108a compares the RTO value
acquired from the RTO request unit 106a with the HO processing time
estimated by the HO time estimator 107a (step S23a). When the RTO value
is larger than the HO processing time (refer to Y route of step S23a),
the line-speed-regulation judgment unit 108a judges that the line speed
regulation is not necessary and requests the HO-execution-confirmation
notifier 103a to transmit an HO-execution-confirmation notification by
notifying it of the above-mentioned MS identifier (MS#01) (step S28a).

[0181]On the other hand, when the RTO value is equal to or smaller than
the HO processing time (see N route of step S23a), the
line-speed-regulation judgment unit 108a judges that the line speed
regulation (lowering control) is necessary, because the timer will exceed
the RTO value during the HO process without it, and requests the line
speed regulator 102a to regulate (perform control to lower) the line
speed by notifying it of the above-mentioned MS identifier (MS#01).

[0182]Upon the reception of the above-mentioned request for line speed
regulation, the line speed regulator 102a creates a control message in
which, as described earlier for FIG. 5, the symbol number and channel
number (both of which are "1" in the present example, for example) are
specified at the data portion and a message type (TYPE#01) expressing
"line speed regulation" and the CID (CID#01) of the corresponding MS 2a,
which is acquired from the MS information storage 152a using the
above-mentioned MS identifier (MS#01) as the search key, are specified at
the header portion, and transmits that control message to the distributor
151a (step S24a, S25a).

[0183]Upon receiving it, the distributor 151a transmits that control
message to the MS 2a via the wireless network (step S26a).

[0184]Subsequently, the line speed regulator 102a requests the
HO-execution-confirmation notifier 103a to transmit an
HO-execution-confirmation notification by notifying it of the
above-mentioned MS identifier (MS#01) (step S27a).

[0185]Upon the reception of the above-mentioned request for transmission
of the HO-execution-confirmation notification from the
line-speed-regulation judgment unit 108a or the line speed regulator
102a, the HO-execution-confirmation notifier 103a creates a control
message in which, as described earlier for FIG. 5, the information
expressing "HO execution enabling" is specified at the data portion and a
message type (TYPE#02) expressing "HO execution confirmation" and the CID
(CID#01) of the corresponding MS 2a, which is acquired from the MS
information storage 152a, are specified at the header portion, and
transmits that control message to the distributor 151a (steps S29a,
S30a).

[0186]Upon receiving it, the distributor 151a transmits that control
message to the MS 2a via the wireless network (step S31a).

[0188]Incidentally, the line speed, which was controlled to be lowered by
the aforementioned procedure before executing the HO, is reassigned to
the MS 2a by the HO-target BS 4a independently of the state before the HO
execution after completion of the HO. Concerning the reassignment method
and the line speed, it is possible that, for example, the HO-target BS 4a
determines the line speed to be reassigned to the MS 2a in accordance
with the number of the currently-processed MSs and notifies the MS 2a of
the determined line speed using a control message (TYPE#01) when the MS
2a communicates with the HO-target BS 4a.

[0189]In this way, the wireless communication system 1a of the present
variation can also increase the RTO value so as to prevent the timer
(retransmission timer) from exceeding the RTO value during the HO process
by performing control to lower the line speed of the communication with
the MS 2a before executing the HO, in the same way as the wireless
communication system 1 of the first embodiment. Therefore, it is possible
to inhibit the throughput reduction caused by a slow-start algorithm
originating from the above-mentioned time-out.

[B] Description of the Second Embodiment

[0190]FIG. 12 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
second embodiment of the present invention. FIG. 13 is a sequence diagram
illustrating an operation (handover process) of the wireless
communication system (especially, MS 2b) illustrated in FIG. 12. FIG. 14
is a sequence diagram illustrating an operation (handover process) of the
wireless communication system (especially, BS 3b) illustrated in FIG. 12.
All of these figures illustrate an embodiment in which the MS judges
whether HO processes are executed or not, in the same way as the first
embodiment.

[0191]Therefore, also in the wireless communication system 1b illustrated
in FIG. 12, at least one MS 2b and a plurality of BSs 3b (including the
HO-target BS 4b) are connected to each other via a wireless network.
However, the present embodiment differs from the construction illustrated
in FIG. 1 mainly in that a range-of-lowering calculator 109b and a
network (NW) information storage 110b are added to the BS 3b.

[0192]Therefore, the MS 2b comprises an HO execution judgment unit 204b,
an HO-target BS decider 206b, an adjacent BS information storage 211b, an
adjacent BS information registrar 253b, an HO-execution-notification
transmitter 207b, an RTO-request receiver 201b, an RTO calculator 202b,
an RTO value storage 252b, an RTO value transmitter 203b, an
HO-execution-confirmation receiver 208b, an HO processor 210b, and a
distributor 251b, which are of the same or similar functions as or to
each corresponding units 204, 206, 211, 253, 207, 201, 202, 252, 203,
208, 210 and 251 described earlier for FIG. 1. The BS 3b comprises an
HO-execution-notification receiver 101b, an HO time estimator 107b, an
RTO request unit 106b, a line-speed-regulation judgment unit 108b, a line
speed regulator 102b, an MS information storage 152b, an adjacent BS
information registrar 153b, an adjacent BS information storage 111b, an
HO-execution-confirmation notifier 103b, and a distributor 151b, which
are of the same or similar functions as or to each corresponding units
101, 106, 107, 108, 102, 152, 153, 111, 103 and 151 described earlier for
FIG. 1, and also comprises a range-of-lowering calculator 109b and an NW
information storage 110b.

[0193](Description of MS 2b)

[0194]In the MS 2b, the HO execution judgment unit 204b, the HO-target BS
decider 206b, the adjacent BS information storage 211b, the adjacent BS
information registrar 253b, the HO-execution-notification transmitter
207b, the RTO-request receiver 201b, the RTO calculator 202b, the RTO
value storage 252b, the RTO value transmitter 203b, the
HO-execution-confirmation receiver 208b, the HO processor 210b, and the
distributor 251b are of the same or similar functions as or to each
corresponding units described earlier for FIG. 1.

[0195]Therefore, the MS 2b according to the second embodiment can also
decide HO execution and request the BS 3b to perform HO upon reception of
a signal from the BS 3b, as well as transmit the RTO value which is
calculated by the RTO calculator 202b to the BS 3b in accordance with a
request from the BS 3b, in the same way as the MS 2 of the first
embodiment. In addition, the MS 2b of the present example can also
perform congestion control (communication speed control) to lower the
line speed of the communication with the communication party upon
time-out of the RTO value.

[0196](Description of BS 3B)

[0197]On the other hand, in the BS 3b, the HO-execution-notification
receiver 101b, the RTO request unit 106b, the HO time estimator 107b, the
line-speed-regulation judgment unit 108b, the adjacent BS information
registrar 153b, the adjacent BS information storage 111b, the MS
information storage 152b, the line speed regulator 102b, the
HO-execution-confirmation notifier 103b, and the distributor 151b are of
the same or similar functions as or to each corresponding units described
earlier for FIG. 1.

[0198]The RTO request unit (timer-value acquisition unit) 106b has the
same function as the RTO request unit 106 described earlier. Namely, the
RTO request unit 106b requests the MS 2b to send the RTO value and
acquires the present RTO value from the MS 2b, with the reception of the
RTO-value acquisition request from the HO-execution-notification receiver
101b as a trigger.

[0199]Namely, the RTO request unit 106b can request the MS 2b to send RTO
by creating a control message (refer to FIG. 5) expressing an "RTO
request" and transmitting the control message to the distributor 151b and
acquire the RTO value from the MS 2b by receiving a control message
(refer to FIG. 5) expressing an "RTO transmission", which is the reply to
the above-mentioned transmitted control message, from the MS 2b. In the
present example, the RTO request unit 106b has an additional function of
registering (storing) the acquired RTO values in the NW information
storage 11b with those RTO values associated with the line speed (channel
capacity) of the communication with the MS 2b at the time of acquiring
each RTO value.

[0200]The NW information storage (history information record unit) 11b,
which is added to the BS 3b in the present example, stores (records) the
RTO values received by the RTO request unit 106b as the history
information about the communication speed with each RTO value associated
with the channel capacity of the communication with the MS 2b at the time
of receiving the RTO value. FIG. 15 illustrates an example of the history
information stored in the NW information storage 110b. In the example
illustrated in FIG. 15, the channel capacities at the time of acquisition
of each RTO value are recorded as the corresponding numbers of channels
used and symbol numbers, with respect to each RTO value (ms) acquired by
the RTO request unit 106b. In the case where the RTO value is equal to 50
ms, the symbol number is "10" and the channel number is "5" then.

[0201]The range-of-lowering calculator 109b, which is also added to the BS
3b in the present example, calculates the range of lowering (lowering
value) of the speed of the communication with the MS 2b via the wireless
network at the time of the lowering control. The range-of-lowering
calculator 109b acquires the line speed (namely, the symbol number and
the channel number) corresponding to the RTO value larger than the HO
processing time from the NW information storage 110b and notifies the
line speed regulator 102b of those symbol number and channel number, with
the reception of the speed-lowering-control request from the
line-speed-regulation judgment unit 108b as a trigger. On the other hand,
when no RTO value that is larger than the HO processing time is
registered in the NW information storage 110b, the range-of-lowering
calculator 109b calculates an appropriate RTO value on the basis of the
history information that is already registered and notifies the line
speed regulator 102b of the calculated symbol number and channel number.

[0202]For example when the HO processing time is 230 ms, the
range-of-lowering calculator 109b can calculate an appropriate RTO value
of 250 ms, by adding the RTO value of 200 ms (whose symbol number is 20
and channel number is 20) and the RTO value of 50 ms (whose symbol number
is 10 and channel number is 5), and notify the line speed regulator 102b
of the symbol number of 30 and the channel number of 25, which correspond
to the calculated RTO value. Namely, the range-of-lowering calculator
109b performs the function of lowering-value calculation unit which
calculates the lowering value of the communication speed (line speed) at
the time of the lowering control on the basis of the history information
recorded in the NW information storage 110b.

[0203]The line speed regulator 102b regulates the speed of the
communication with the MS 2b via the wireless network. The line speed
regulator 102b regulates (performs control to lower) the speed (namely,
the symbol number and the channel number) of the communication with the
MS 2b via the wireless network on the basis of the symbol number and the
channel number that are sent from the above-mentioned range-of-lowering
calculator 109b, with the reception of the symbol number and the channel
number sent from the above-mentioned range-of-lowering calculator 109b as
a trigger. Namely, the line speed regulator 102b creates a control
message (refer to FIG. 5) expressing a "line speed regulation" and
transmits the control message to the MS 2b via the distributor 151b. When
the control of communication speed is completed, the line speed regulator
102b requests the HO-execution-confirmation notifier 103b to transmit an
HO-execution-confirmation notification.

[0204]In this way, the BS 3b according to the second embodiment of the
present invention can create, if necessary, a simulated line congestion
through a control to lower the speed (namely, the symbol number and the
channel number) of the communication with the MS 2b during the HO
processing by an appropriate range of lowering on the basis of the
above-mentioned history information.

[0205](Description of Entire Operation of Wireless Communication System
1b)

[0206]Next, an example of the entire operation of the wireless
communication system 1b having the above-mentioned structure will be
described with reference to FIG. 13 and FIG. 14.

[0207]First, the MS 2b acquires the CID (for example, CID#01) of the
connection to the serving BS 3b in advance. The CID (CID#01) is
controlled by the distributor 251b.

[0208]In addition, in the MS 2b, the adjacent BS information registrar
253b acquires information (adjacent BS information) about the BSs 3b
(including the HO-target BS 4b) that are geographically neighboring on
the serving BS 3b, such as their frequencies used, capability types
supported (typically including security function, packet retransmission
function, MIMO function, etc.), radio field strengths, and the number of
MSs in charge, and registers it in the adjacent BS information storage
211b (step S1b). The MS 2b can acquire that adjacent BS information by,
for example, receiving it when it is transmitted (reported) from the BS
3b.

[0209]Moreover, in the MS 2b, the RTO calculator 202b measures the RTTs of
the TCP data that are transmitted/received to/from the communication
parties (namely, other MSs and BS 3b) regularly or irregularly and
calculates the RTO values from those RTTs so as to store them in the RTO
value storage 252b (step S2b).

[0210]On the other hand, the BS 3b acquires information (MS information)
about the MS 2b such as the identifier of the MS 2b (for example, MS#01)
and capability types supported by the MS 2b (typically including security
function, packet retransmission function, MIMO function, etc.), and
registers it in the MS information storage 152b. That MS information can
be acquired by, for example, transmission of necessary information also
from the MS 2b at the phase when the MS 2b starts to communicate with the
BS 3b.

[0211]Next, in the BS 3b, the adjacent BS information registrar 153b
acquires information (adjacent BS information) about the geographically
neighboring BSs 3b (including the HO-target BS 4b) in advance, such as
the frequency used and the capability types supported, and registers it
in the adjacent BS information storage 111b (step S3b). That adjacent BS
information can be acquired by, for example, exchanging information with
the neighboring BSs 3b (including the HO-target BS 4b) regularly or
irregularly with the above-mentioned CIDs to be used for exchanging BS
information.

[0212]After the above-mentioned preliminary processes such as registration
of information in the MS 2b and the BS 3b, the MS 2b checks whether an HO
should be executed or not by measuring the radio field strength of the
serving BS 3b regularly or irregularly by the HO execution judgment unit
204b. When the radio field strength is lower than the value that was
preset by the administrator with a command, the HO execution judgment
unit 204b judges that an HO execution is necessary and requests the
HO-target BS decider 206b to decide the HO-target BS 4b (step S4b).

[0213]The HO-target BS decider 206b refers to the adjacent BS information
storage 211b, with the above-mentioned request from the HO execution
judgment unit 204b as a trigger, and determines the HO-target BS 4b on
the basis of the adjacent BS information (step S5b). For example, the BS
3b of which radio field strength is the highest of the neighboring BSs 3b
is determined to be the HO-target BS 4b. Then, the HO-target BS decider
206b notifies the HO-execution-notification transmitter 207b of the BS
identifier (for example, BS#02) of the determined HO-target BS 4b, and
also, requests the HO-execution-notification transmitter 207b to transmit
an HO execution notification (step S6b).

[0214]The HO-execution-notification transmitter 207b creates a control
message, upon reception of the above-mentioned request from the HO-target
BS decider 206b, in which the message type (TYPE#00) expressing an "HO
execution notification" is specified at the header portion and the BS
identifier (BS#02) of the HO-target BS 4b and the MS identifier (MS#01)
of the MS 2b, of which communication process is going to be handed over,
are specified at the data portion and transmits the control message to
the distributor 251b (step S7b).

[0215]The distributor 251b specifies the CID (CID#01) at the header
portion of that control message and transmits that control message to the
BS 3b via the wireless network (step S8b).

[0216]In the BS 3b, the HO-execution-notification receiver 101b receives
that control message (HO execution notification) via the distributor
151b, which is transmitted from the MS 2b (step S9b), acquires the MS
identifier (MS#01) and the BS identifier (BS#02) contained in the data
portion of that HO execution notification, and then sends a request for
estimation of the HO processing time to the HO time estimator 107b
together with the BS identifier (step S10b). In addition, the
HO-execution-notification receiver 101b acquires the MS identifier
(MS#01) from the data portion of that control message and notifies the HO
time estimator 107b of the MS identifier (MS#O1).

[0217]The HO time estimator 107b acquires the MS information, from the MS
information storage 152b, about the MS 2b that transmitted the HO
execution notification on the basis of the above-mentioned MS identifier
(MS#01) (step 11b), and transmits information about the functions
supported by the MS 2b to the BS 3b (HO-target BS 4b) of which identifier
is the above-mentioned BS identifier (BS#02). In addition, the HO time
estimator 107b estimates time (HO processing time) required for
negotiation between the MS 2b and the BS 3b at the time of HO process on
the basis of the information about the negotiation which is stored in the
adjacent BS information storage 111b, and transmits the estimated HO
processing time to the line speed regulator 108b. Moreover, the HO time
estimator 107b notifies the RTO request unit 106b of an RTO acquisition
request in order to acquire the RTO value of the MS 2b corresponding to
the MS identifier that is acquired from the HO-execution-notification
receiver 101b (step S12b).

[0218]The RTO request unit 106b creates a control message expressing "RTO
acquisition" when the RTO request unit 106b receives the above-mentioned
RTO-value acquisition request from the HO time estimator 107b. For
example, the RTO request unit 106b searches the MS information storage
152b using the above-mentioned MS identifier (MS#01) as a search key so
as to acquire the CID (CID#01) of the corresponding MS 2b (step S13b),
and specifies that CID at the header portion of the control message, as
well as specifies a message type (TYPE#03) expressing "RTO acquisition"
at the header portion. Then the RTO request unit 106b transmits the
control message to the distributor 151b (step S14b).

[0219]Upon receiving it, the distributor 151b transmits that control
message to the MS 2b via the wireless network (step S15b).

[0220]In the MS 2b, the RTO-requirement receiver 201b receives the
above-mentioned control message expressing "RTO acquisition" from the BS
3b via the distributor 251b (step S16b), acquires the present RTO value
from the RTO value storage 252b (step S17b), and requests the RTO value
transmitter 203b to transmit the RTO value while notifying it of the RTO
value (step S18b).

[0221]The RTO value transmitter 203b creates a control message by
specifying the RTO value sent from the RTO-requirement receiver 201b at
the data portion and specifying a message type (TYPE#04) expressing "RTO
transmission" at the header portion, and transmits that control message
to the distributor 251b (step S19b).

[0222]Upon receiving it, the distributor 251b adds the CID (CID#01) at the
header portion of that control message and transmits that control message
to the BS 3b via the wireless network (step S20b).

[0223]Next, in the BS 3b, the RTO request unit 106b receives the
above-mentioned control message expressing "RTO transmission" (step
S21b), acquires the RTO value specified at the data portion of the
message, registers (stores) the acquired RTO value in the NW information
storage 110b by associating it with the speed (channel capacity) of the
communication with the MS 2b at the time of acquisition of it (step
S34b), and transmits the acquired RTO value to the line-speed-regulation
judgment unit 108b (step S22b).

[0224]The line-speed-regulation judgment unit 108b compares the RTO value
acquired from the RTO request unit 106b with the HO processing time
estimated by the HO time estimator 107b (step S23b). When the RTO value
is larger than the HO processing time (see Y route of step S23b), the
line-speed-regulation judgment unit 108b judges that the line speed
regulation is not necessary and requests the HO-execution-confirmation
notifier 103b to transmit an HO-execution-confirmation notification by
notifying it of the above-mentioned MS identifier (MS#01) (step S28b).

[0225]On the other hand, when the RTO value is equal to or smaller than
the HO processing time (see N route of step S23b), the
line-speed-regulation judgment unit 108b judges that the line speed
regulation (lowering control) is necessary, because the timer will exceed
the RTO value during the HO process without it, and requests the
range-of-lowering calculator 109b to calculate the regulation value
(range of lowering) of the line speed by notifying it of the
above-mentioned MS identifier (MS#01) (step S35b).

[0226]Upon receiving the above-mentioned request for calculation, the
range-of-lowering calculator 109b acquires (or calculates) the symbol
number and the channel number that correspond to the smallest RTO value
of those larger than the HO processing time estimated by the HO time
estimator 107b from the NW information storage 110b (step S36b). Then,
the range-of-lowering calculator 109b requests the line speed regulator
102b to regulate (perform control to lower) the line speed by notifying
it of the above-mentioned MS identifier (MS#01) and the above-mentioned
symbol number and channel number (step S37b).

[0227]Upon the reception of the above-mentioned request for line speed
regulation, the line speed regulator 102b creates a control message in
which, as described earlier for FIG. 5, the symbol number and channel
number (the symbol number and channel number sent from the
range-of-lowering calculator 109b, in the present example) are specified
at the data portion and a message type (TYPE#01) expressing "line speed
regulation" and the CID (CID#01) of the corresponding MS 2b, which is
acquired from the MS information storage 152b using the above-mentioned
MS identifier (MS#01) as the search key, are specified at the header
portion, and transmits that control message to the distributor 151b
(steps S24b, S25b).

[0228]Upon receiving it, the distributor 151b transmits that control
message to the MS 2b via the wireless network (step S26b).

[0229]Subsequently, the line speed regulator 102b requests the
HO-execution-confirmation notifier 103b to transmit an
HO-execution-confirmation notification by notifying it of the
above-mentioned MS identifier (MS#01) (step S27b).

[0230]Upon the reception of the above-mentioned request for transmission
of the HO-execution-confirmation notification from the
line-speed-regulation judgment unit 108b or the line speed regulator
102b, the HO-execution-confirmation notifier 103b creates a control
message in which, as described earlier for FIG. 5, the information
expressing "HO execution enabling" is specified at the data portion and a
message type (TYPE#02) expressing "HO execution confirmation" and the CID
(CID#01) of the corresponding MS 2b, which is acquired from the MS
information storage 152b, are specified at the header portion, and
transmits that control message to the distributor 151b (steps S29b,
S30b).

[0231]Upon receiving it, the distributor 151b transmits that control
message to the MS 2b via the wireless network (step S31b).

[0233]Incidentally, the line speed, which was controlled to be lowered by
the aforementioned procedure before executing the HO, is reassigned to
the MS 2b by the HO-target BS 4b independently of the state before the HO
execution after completion of the HO. Concerning the reassignment method
and the line speed, it is possible that, for example, the HO-target BS 4b
determines the line speed to be reassigned to the MS 2b in accordance
with the number of the currently-processed MSs and notifies the MS 2b of
the determined line speed using a control message (TYPE#01) when the MS
2b communicates with the HO-target BS 4b.

[0234]As described above, according to the second embodiment of the
present invention, when an HO execution is decided by the MS 2b and the
requirement for it is sent to the BS 3b, the BS 3b calculates an
appropriate line speed (calculates the range of lowering) so that the RTO
value is larger than the HO processing time then, on the basis of the
history information about the communication speed, and performs control
to lower the line speed of the communication with the MS 2b
appropriately, before executing the HO. This can create a simulated
congestion of the line and increase the RTO value to the degree that the
RTO value is longer than the time that is necessary for completion of the
HO process.

[0235]Therefore, in addition to the operation and effect that are the same
as those of the first embodiment, it is possible to prevent the timer
(retransmission timer) from exceeding the RTO value without enlarging the
RTO value unnecessarily, because the RTO value is controlled to be the
smallest of those larger than the time required for completion of the HO
by calculating the range of lowering.

[B1] Description of the Variation of the Second Embodiment

[0236]In the above-mentioned second embodiment, explanation was given on
the case where the necessities of HO processes are checked and decided by
the MS 2b, in other words, the MS 2b takes the initiative to execute HO
processes. However, the BS 3b may take the initiative to execute HO
processes. Such an embodiment will be described in the explanation of the
present variation example with reference to FIG. 16 to FIG. 18.

[0237]FIG. 16 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
variation of the above-mentioned second embodiment. FIG. 17 is a sequence
diagram illustrating an operation (handover process) of the wireless
communication system (especially, the MS 2c) illustrated in FIG. 16. FIG.
18 is a sequence diagram illustrating an operation (handover process) of
the wireless communication system (especially, the BS 3c) illustrated in
FIG. 16.

[0238]Namely, in the wireless communication system 1c illustrated in FIG.
16, at least one MS 2c and a plurality of BSs 3c (including the HO-target
BS 4c) are connected to each other via a wireless network, in the same
way as the wireless communication system 1b of the second embodiment.
However, it differs from the wireless communication system 1b of the
first embodiment in that the BS 3c decides whether HO processes will be
executed or not.

[0239](Description of MS 2c)

[0240]Therefore, the MS 2c of the present example comprises an adjacent BS
information storage 211c, an adjacent BS information registrar 253c, an
RTO-request receiver 201c, an RTO calculator 202c, an RTO value storage
252c, an RTO value transmitter 203c, an HO-execution-confirmation
receiver 208c, an HO processor 210c, and a distributor 251c, which are of
the same or similar functions as or to each corresponding units 211b,
253b, 201b, 202b, 252b, 203b, 208b, 210b and 251b described earlier for
FIG. 12 to FIG. 14, and it may not need the functions of the HO execution
judgment unit 204b and the HO-target BS decider 206b, which were
illustrated in FIG. 12.

[0241](Description of BS 3c)

[0242]On the other hand, the BS 3c of the present example comprises, an
RTO request unit 106c, an HO time estimator 107c, a line-speed-regulation
judgment unit 108c, a range-of-lowering calculator 109c, an NW
information storage 110c, an adjacent BS information registrar 153c, an
adjacent BS information storage 111c, an MS information storage 152c, a
line speed regulator 102c, an HO-execution-confirmation notifier 103c,
and a distributor 151c, which are of the same or similar functions as or
to each corresponding units described earlier for FIG. 12, and also
comprises an HO execution judgment unit 104c and an HO-target BS decider
105c, in place of the above-mentioned HO-execution-notification receiver
101b described earlier for FIG. 12.

[0243]The HO execution judgment unit 104c of the BS 3c collects
information about the execution condition of HO processes and judges
whether the HO processes will be executed or not. When the HO is
executed, the HO execution judgment unit 104c notifies the HO-target BS
decider 105c of the HO execution and requests the HO-target BS decider
105c to decide the HO-target BS 4c. Namely, the HO execution judgment
unit 104c performs one of the functions of the handover decision means
which autonomously decides execution of HO of the communication with the
MS 2c to another BS 3c (HO-target BS 4c).

[0244]The HO-target BS decider 105c decides the HO-target BS 4c on the
basis of the adjacent BS information stored in the adjacent BS
information storage 111c by selecting from the adjacent BSs 3c that are
already registered, with the reception of the request for decision of the
HO-target BS 4c from the HO execution judgment unit 104c as a trigger.
Concerning the criterion of deciding (selecting) the BS, the BS 3c whose
radio field strength is higher than those of the other BSs 3c can be
selected (decided) as the HO-target BS 4c, for example, also in the
present variation.

[0245](Description of Entire Operation of Wireless Communication System
1c)

[0246]In the following, an example of the entire operation of the wireless
communication system 1c of which construction is such as described above
is illustrated in FIG. 17 and FIG. 18.

[0247]First, the MS 2c acquires the CID (for example, CID#O1) of the
connection to the serving BS 3c in advance. The CID (CID#01) is
controlled by the distributor 251c.

[0248]In addition, in the MS 2c, the adjacent BS information registrar
253c acquires adjacent BS information about the BSs 3c (including the
HO-target BS 4c) that are geographically neighboring on the serving BS
3c, and registers it in the adjacent BS information storage 211c (step
S1c), as illustrated in FIG. 17. Moreover, in the MS 2c, the RTO
calculator 202c measures the RTTs of the TCP data that are
transmitted/received to/from the communication parties (namely, other MSs
and Bss 3c) regularly or irregularly and calculates the RTO values from
those RTTs so as to store them in the RTO value storage 252c (step S2c).

[0249]On the other hand, the BS 3c acquires information (MS information)
about the MS 2c, such as the identifier of the MS 2c (for example, MS#01)
and capability types supported by the MS 2c (typically including security
function, packet retransmission function, MIMO function, etc.), and
registers it in the MS information storage 152c. That MS information can
be acquired by, for example, transmission of necessary information also
from the MS 2c at the phase when the MS 2c starts to communicate with the
BS 3c.

[0250]Next, in the BS 3c, the adjacent BS information registrar 153c
acquires the adjacent BS information about the geographically neighboring
BSs 3c (including the HO-target BS 4c) in advance, and registers it in
the adjacent BS information storage 111c (step S3c). That adjacent BS
information can be acquired by, for example, exchanging information with
the neighboring BSs 3c (including the HO-target BS 4c) regularly or
irregularly with the above-mentioned CIDs to be used for exchanging BS
information.

[0251]After the above-mentioned preliminary processes such as registration
of information in the MS 2c and the BS 3c, the BS 3c checks whether an HO
should be executed or not by measuring the radio field strength of the MS
2c regularly or irregularly by the HO execution judgment unit 104c. When
the radio field strength is lower than the value that was preset by the
administrator with a command, the HO execution judgment unit 104c judges
that an HO execution is necessary and requests the HO-target BS decider
106c[sic] to decide the HO-target BS 4c (step S4c).

[0252]The HO-target BS decider 106c[sic] refers to the adjacent BS
information storage 111c, with the above-mentioned request from the HO
execution judgment unit 104c as a trigger, and determines the HO-target
BS 4c on the basis of the adjacent BS information (step S5c). For
example, the BS 3c of which radio field strength is the highest of the
neighboring BSs 3c is determined to be the HO-target BS 4c. Then, the
HO-target BS decider 106c[sic] sends a request for estimation of the HO
processing time to the HO time estimator 107c together with the BS
identifier (BS#02) of the HO-target BS 4c (step S10c). In addition, the
HO-target BS decider 106c[sic] notifies the HO time estimator 107c of the
MS identifier (MS#01) corresponding to the MS 2c of which communication
is going to be handed over.

[0253]The HO time estimator 107c acquires the MS information, from the MS
information storage 152c, about the corresponding MS 2c on the basis of
the above-mentioned MS identifier (MS#01) (step 11c), and transmits
information about the functions supported by the MS 2c to the BS 3c
(HO-target BS 4c) that possesses the above-mentioned BS identifier
(BS#02). In addition, the HO time estimator 107c estimates time (Ho
processing time) required for negotiation between the MS 2c and the BS 3c
at the time of HO process on the basis of the information about the
negotiation which is stored in the adjacent BS information storage 111c,
and transmits the estimated HO processing time to the line speed
regulator 108c.

[0254]The RTO request unit 106c creates a control message expressing "RTO
acquisition" when the RTO request unit 106c receives the above-mentioned
RTO-value acquisition request from the HO time estimator 107c (step
S12c). For example, the RTO request unit 106c searches the MS information
storage 152c using the above-mentioned MS identifier (MS#01) as a search
key so as to acquire the CID (CID#01) of the corresponding MS 2c (step
S13c), and specifies that CID at the header portion of the control
message, as well as specifies the message type (TYPE#03) expressing "RTO
acquisition" at the header portion. Then the RTO request unit 106c
transmits the control message to the distributor 151c (step S14c).

[0255]Upon receiving it, the distributor 151c transmits that control
message to the MS 2c via the wireless network (step S15c).

[0256]In the MS 2c, the RTO-requirement receiver 201c receives the
above-mentioned control message expressing "RTO acquisition" from the BS
3c via the distributor 251c (step S16c), acquires the present RTO value
from the RTO value storage 252c (step S17c), and requests the RTO value
transmitter 203c to transmit the RTO value while notifying it of the RTO
value (step S18c).

[0257]The RTO value transmitter 203c creates a control message by
specifying the RTO value sent from the RTO-requirement receiver 201c at
the data portion and specifying a message type (TYPE#04) expressing "RTO
transmission" at the header portion, and transmits that control message
to the distributor 251c (step S19c).

[0258]Upon receiving it, the distributor 251c adds the CID (CID#01) at the
header portion of that control message and transmits that control message
to the BS 3c via the wireless network (step S20c).

[0259]Next, in the BS 3c, the RTO request unit 106c receives the
above-mentioned control message expressing "RTO transmission" (step
S21c), acquires the RTO value specified at the data portion of the
message, registers (stores) the acquired RTO value in the NW information
storage 110c by associating it with the speed (channel capacity) of the
communication with the MS 2c at the time of acquisition of it (step
S34c), and transmits the acquired RTO value to the line-speed-regulation
judgment unit 108c (step S22c).

[0260]The line-speed-regulation judgment unit 108c compares the RTO value
acquired from the RTO request unit 106c with the HO processing time
estimated by the HO time estimator 107c (step S23c). When the RTO value
is larger than the HO processing time (see Y route of step S23c), the
line-speed-regulation judgment unit 108c judges that the line speed
regulation is not necessary and requests the HO-execution-confirmation
notifier 103c to transmit an HO-execution-confirmation notification by
notifying it of the above-mentioned MS identifier (MS#01) (step S28c).

[0261]On the other hand, when the RTO value is equal to or smaller than
the HO processing time (see N route of step S23c), the
line-speed-regulation judgment unit 108c judges that the line speed
regulation (lowering control) is necessary, because the timer will exceed
the RTO value during the HO process without it, and requests the
range-of-lowering calculator 109c to calculate the regulation value
(range of lowering) of the line speed by notifying it of the
above-mentioned MS identifier (MS#01) (step S35c).

[0262]Upon receiving the above-mentioned request for calculation, the
range-of-lowering calculator 109c acquires (or calculates) the symbol
number and the channel number that correspond to the smallest RTO value
of those larger than the HO processing time estimated by the HO time
estimator 107c from the NW information storage 110c (step S36c). Then,
the range-of-lowering calculator 109c requests the line speed regulator
102c to regulate (perform control to lower) the line speed by notifying
it of the above-mentioned MS identifier (MS#01) and the above-mentioned
symbol number and channel number (step S37c).

[0263]Upon the reception of the above-mentioned request for line speed
regulation, the line speed regulator 102c creates a control message in
which, as described earlier for FIG. 5, the symbol number and channel
number (the symbol number and channel number sent from the
range-of-lowering calculator 109c, in the present example) are specified
at the data portion and a message type (TYPE#01) expressing "line speed
regulation" and the CID (CID#01) of the corresponding MS 2c, which is
acquired from the MS information storage 152c using the above-mentioned
MS identifier (MS#O1) as the search key, are specified at the header
portion, and transmits that control message to the distributor 151c
(steps S24c, S25c).

[0264]Upon receiving it, the distributor 151c transmits that control
message to the MS 2c via the wireless network (step S26c).

[0265]Subsequently, the line speed regulator 102c requests the
HO-execution-confirmation notifier 103c to transmit an
HO-execution-confirmation notification by notifying it of the
above-mentioned MS identifier (MS#01) (step S27c).

[0266]Upon the reception of the above-mentioned request for transmission
of the HO-execution-confirmation notification from the
line-speed-regulation judgment unit 108c or the line speed regulator
102c, the HO-execution-confirmation notifier 103c creates a control
message in which, as described earlier for FIG. 5, the information
expressing "HO execution enabling" is specified at the data portion and a
message type (TYPE#02) expressing "HO execution confirmation" and the CID
(CID#01) of the corresponding MS 2c, which is acquired from the MS
information storage 152c, are specified at the header portion, and
transmits that control message to the distributor 151c (steps S29c,
S30c).

[0267]Upon receiving it, the distributor 151c transmits that control
message to the MS 2c via the wireless network (step S31c).

[0269]Incidentally, the line speed, which was controlled to be lowered by
the aforementioned procedure before executing the HO, is reassigned to
the MS 2c by the HO-target BS 4c independently of the state before the HO
execution after completion of the HO. Concerning the reassignment method
and the line speed, it is possible that, for example, the HO-target BS 4c
determines the line speed to be reassigned to the MS 2c in accordance
with the number of the currently-processed MSs and notifies the MS 2c of
the determined line speed using a control message (TYPE#01) when the MS
2c communicates with the HO-target BS 4c.

[0270]In this way, the wireless communication system 1c of the present
variation can also increase the RTO value so as to prevent the timer
(retransmission timer) from exceeding the RTO value by performing control
to lower the line speed of the communication with the MS 2c before
executing the HO, in the same way as the wireless communication system 1b
of the second embodiment. Therefore, it is possible to inhibit the
throughput reduction caused by a slow-start algorithm originating from
the above-mentioned time-out without inducing throughput reduction in
normal times. In addition, it is possible to prevent the timer
(retransmission timer) from exceeding the RTO value without enlarging the
RTO value unnecessarily, because the RTO value is controlled to be the
smallest of those larger than the time required for completion of the HO
by calculating the range of lowering.

[B] Description of the Third Embodiment

[0271]FIG. 19 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
third embodiment of the present invention. FIG. 20 is a sequence diagram
illustrating an operation (handover process) of the wireless
communication system (especially, MS 2d) illustrated in FIG. 19. FIG. 21
is a sequence diagram illustrating an operation (handover process) of the
wireless communication system (especially, BS 3d) illustrated in FIG. 19.
All of these figures illustrate an embodiment in which the MS judges
whether HO processes are executed or not, in the same way as the first
and second embodiments.

[0272]Therefore, also in the wireless communication system 1d illustrated
in FIG. 19, at least one MS 2d and a plurality of BSs 3d (including the
HO-target BS 4d) are connected to each other via a wireless network.
However, the present embodiment differs from the construction illustrated
in FIG. 1 mainly in that the judgment and the regulation of the line
speed are carried out repeatedly.

[0273]Therefore, the MS 2d comprises an HO execution judgment unit 204d,
an HO-target BS decider 206d, an adjacent BS information storage 211d, an
adjacent BS information registrar 253d, an HO-execution-notification
transmitter 207d, an RTO-request receiver 201d, an RTO calculator 202d,
an RTO value storage 252d, an RTO value transmitter 203d, an
HO-execution-confirmation receiver 208d, an HO processor 210d, and a
distributor 251d, which are of the same or similar functions as or to
each corresponding units 204, 206, 211, 253, 207, 201, 202, 252, 203,
208, 210 and 251 described earlier for FIG. 1. The BS 3d comprises an
HO-execution-notification receiver 101d, an HO time estimator 107d, an
RTO request unit 106d, a line-speed-regulation judgment unit 108d, a line
speed regulator 102d, an MS information storage 152d, an adjacent BS
information registrar 153d, an adjacent BS information storage 111d, an
HO-execution-confirmation notifier 103d, and a distributor 151d, which
are of the same or similar functions as or to each corresponding units
101, 106, 107, 108, 102, 152, 153, 111, 103 and 151 described earlier for
FIG. 1.

[0274](Description of MS 2d)

[0275]In the MS 2d, the HO execution judgment unit 204d, the HO-target BS
decider 206d, the adjacent BS information storage 211d, the adjacent BS
information registrar 253d, the HO-execution-notification transmitter
207d, the RTO-request receiver 201d, the RTO calculator 202d, the RTO
value storage 252d, the RTO value transmitter 203d, the
HO-execution-confirmation receiver 208d, the HO processor 210d, and the
distributor 251d are of the same or similar functions as or to each
corresponding units described earlier for FIG. 1.

[0276]Therefore, the MS 2d according to the second[sic] embodiment can
also decide HO execution and request the BS 3d to perform HO upon
reception of a signal from the BS 3d, as well as transmit the RTO value
which is calculated by the RTO calculator 202d to the BS 3d in accordance
with a request from the BS 3d, in the same way as the MS 2. In addition,
the MS 2d of the present example can also perform congestion control
(communication speed control) to lower the line speed of the
communication with the communication party upon time-out of the RTO
value.

[0277](Description of BS 3D)

[0278]On the other hand, in the BS 3d, the HO-execution-notification
receiver 101d, the RTO request unit 106d, the HO time estimator 107d, the
line-speed-regulation judgment unit 108d, the adjacent BS information
registrar 153d, the adjacent BS information storage 111d, the MS
information storage 152d, the line speed regulator 102d, the
HO-execution-confirmation notifier 103d, and the distributor 151d are of
the same or similar functions as or to each corresponding units described
earlier for FIG. 1.

[0279]The line-speed-regulation judgment unit 108d of the present example
also judges whether an HO can be executed or not on the basis of the
information (information about the execution condition) stored in the
adjacent BS information storage 111d, and the MS information storage 152d
and judges whether an HO can be executed or not also on the basis of the
RTO value sent by the RTO request unit 106d and the HO processing time
estimated by the HO time estimator 107d. When the HO can be executed, the
line-speed-regulation judgment unit 108d decides the HO execution and
judges whether the line speed will be regulated or not.

[0280]On the other hand, the line speed regulator 102d of the present
example regulates the speed of the communication with the MS 2d via the
wireless network. The line speed regulator 102d performs control to lower
the speed of the communication with the MS 2 via the wireless network at
a preset rate (or by a specified range of lowering), with the reception
of the speed-lowering-control request from the above-mentioned
line-speed-regulation judgment unit 108d as a trigger, and subsequently,
requests the RTO request unit 108d[sic] to reacquire the RTO value.

[0281]The RTO request unit (timer-value acquisition unit) 106d requests
the MS 2d to send the RTO value again and acquires the present RTO value
from the MS 2d, when the RTO request unit 106d receives the RTO-value
acquisition request from the above-mentioned line speed regulator 102d.
Namely, the RTO request unit 106d can request the MS 2d to send an RTO by
creating a control message (refer to FIG. 5) expressing an "RTO request"
and transmitting the control message to the distributor 151d. Also, the
RTO request unit 106d can acquire the RTO value from the MS 2d by
receiving a control message (refer to FIG. 5) expressing an "RTO
transmission", which is the reply to the above-mentioned transmitted
control message, from the MS 2d.

[0282]Then, the line-speed-regulation judgment unit 108d judges whether an
HO can be executed or not on the basis of the present RTO value sent by
the RTO request unit 106d and the HO processing time estimated by the HO
time estimator 107d again. When the HO can be executed, the
line-speed-regulation judgment unit 108d decides the HO execution and
judges whether the line speed will be regulated or not.

[0283]Subsequently, the HO-execution-confirmation notifier 103d transmits
an HO-execution-confirmation notification (enabling acknowledgement),
which expresses an HO-execution enabling, to the MS 2d using a control
message (refer to FIG. 5), with the notification (request) sent from the
line speed regulator 102d as a trigger. Also in the present example, the
HO-execution-confirmation notifier 103d creates a control message in
which, as described earlier for FIG. 5, the CID acquired from the MS
information storage 152d and the message type are specified at the header
portion and the HO-execution enabling (OK) information is specified at
the data portion, with the notification sent from the line speed
regulator 102d as a trigger, and transmits it to the distributor 151d.

[0284]In this way, the BS 3d according to the third embodiment of the
present invention can execute an HO process with the RTO value increased
in order for the timer not to exceed the RTO value by creating simulated
line congestion through repeated controls to lower the speed of the
communication with the MS 2d, if it is necessary. This makes it possible
to perform the above-mentioned control to lower the communication speed
more minutely, and therefore, the RTO value can be controlled to be
appropriate without being enlarged unnecessarily.

[0285](Description of Entire Operation of Wireless Communication System
1d)

[0286]Next, an example of the entire operation of the wireless
communication system 1d having the above-mentioned structure will be
described with reference to FIG. 20 and FIG. 21.

[0287]First, the MS 2d acquires the CID (for example, CID#01) of the
connection to the serving BS 3d in advance. The CID (CID#01) is
controlled by the distributor 251d.

[0288]In addition, in the MS 2d, the adjacent BS information register 253d
acquires information (adjacent BS information) about the BSs 3d
(including the HO-target BS 4d) that are geographically neighboring on
the serving BS 3d, such as their frequencies used, capability types
supported (typically including security function, packet retransmission
function, MIMO function, etc.), radio field strengths, and the number of
MSs in charge, and registers it in the adjacent BS information storage
211d (step S1d). The MS 2d can acquire that adjacent BS information by,
for example, receiving it when it is transmitted (reported) from the BS
3d.

[0289]Moreover, in the MS 2d, the RTO calculator 202d measures the RTTs of
the TCP data that are transmitted/received to/from the communication
parties (namely, other MSs and Bss 3d) regularly or irregularly and
calculates the RTO values from those RTTs so as to store them in the RTO
value storage 252d (step S2d).

[0290]On the other hand, the BS 3d acquires information (MS information)
about the MS 2d such as MS 2d identifier (for example, MS#01) and
capability types supported by the MS 2d (typically including security
function, packet retransmission function, MIMO function, etc.), and
registers it in the MS information storage 152d. That MS information can
be acquired by, for example, transmission of necessary information also
from the MS 2d at the phase when the MS 2d starts to communicate with the
BS 3d.

[0291]Next, in the BS 3d, the adjacent BS information registrar 153d
acquires information (adjacent BS information) about the geographically
neighboring BSs 3d (including the HO-target BS 4d) in advance, such as
the frequency used and the capability types supported, and registers it
in the adjacent BS information storage 111d (step S3d). That adjacent BS
information can be acquired by, for example, exchanging information with
the neighboring BSs 3d (including the HO-target BS 4d) regularly or
irregularly with the above-mentioned CIDs to be used for exchanging BS
information.

[0292]After the above-mentioned preliminary processes such as registration
of information in the MS 2d and the BS 3d, the MS 2d checks whether an HO
should be executed or not by measuring the radio field strength of the
serving BS 3d regularly or irregularly by the HO execution judgment unit
204d. When the radio field strength is lower than the value that was
preset by the administrator with a command, the HO execution judgment
unit 204d judges that an HO execution is necessary and requests the
HO-target BS decider 206d to decide the HO-target BS 4d (step S4d).

[0293]The HO-target BS decider 206d refers to the adjacent BS information
storage 211d, with the above-mentioned request from the HO execution
judgment unit 204d as a trigger, and determines the HO-target BS 4d on
the basis of the adjacent BS information (step S5d). For example, the BS
3d of which radio field strength is the highest of the neighboring BSs 3d
is determined to be the HO-target BS 4d. Then, the HO-target BS decider
206d notifies the HO-execution-notification transmitter 207d of the BS
identifier (for example, BS#02) of the determined HO-target BS 4d, and
also, requests the HO-execution-notification transmitter 207d to transmit
an HO execution notification (step S6d).

[0294]The HO-execution-notification transmitter 207d creates a control
message, upon reception of the above-mentioned request from the HO-target
BS decider 206d, in which the message type (TYPE#00) expressing an "HO
execution notification" is specified at the header portion and the BS
identifier (BS#02) of the HO-target BS 4d and the MS identifier (MS#01)
of the MS 2d, of which communication process is going to be handed over,
are specified at the data portion and transmits the control message to
the distribution unit 251d (step S7d).

[0295]The distributor 251d specifies the CID (CID#01) at the header
portion of that control message and transmits that control message to the
BS 3d via the wireless network (step S8d).

[0296]In the BS 3d, the HO-execution-notification receiver 101d receives
that control message (HO execution notification) via the distributor
151d, which is transmitted from the MS 2d (step S9d), acquires the MS
identifier (MS#01) and the BS identifier (BS#02) contained in the data
portion of that HO execution notification, and then sends a request for
estimation of the HO processing time to the HO time estimator 107d
together with the BS identifier (step S10d). In addition, the
HO-execution-notification receiver 101d acquires the MS identifier
(MS#01) from the data portion of that control message and notifies the HO
time estimator 107d of the MS identifier (MS#O1).

[0297]The HO time estimator 107d acquires the MS information, from the MS
information storage 152d, about the MS 2d that transmitted the HO
execution notification on the basis of the above-mentioned MS identifier
(MS#01) (step S11d), and transmits information about the functions
supported by the MS 2d to the BS 3d (HO-target BS 4d) of which identifier
is the above-mentioned BS identifier (BS#02). In addition, the HO time
estimator 107d estimates time (HO processing time) required for
negotiation between the MS 2d and the BS 3d at the time of HO process on
the basis of the information about the negotiation which is stored in the
adjacent BS information storage 111d, and transmits the estimated HO
processing time to the line speed regulator 108d. Moreover, the HO time
estimator 107d notifies the RTO request unit 106d of an RTO acquisition
request in order to acquire the RTO value of the MS 2d corresponding to
the MS identifier that is acquired from the HO-execution-notification
receiver 101d (step S12d).

[0298]The RTO request unit 106d creates a control message expressing "RTO
acquisition" when the RTO request unit 106d receives the above-mentioned
RTO-value acquisition request from the HO time estimator 107d. For
example, the RTO request unit 106d searches the MS information storage
152d using the above-mentioned MS identifier (MS#01) as a search key so
as to acquire the CID (CID#01) of the corresponding MS 2d (step S13d),
and specifies that CID at the header portion of the control message, as
well as specifies a message type (TYPE#03) expressing "RTO acquisition"
at the header portion. Then the unit 106d transmits the control message
to the distributor 151d (step S14d).

[0299]Upon receiving it, the distributor 151d transmits that control
message to the MS 2d via the wireless network (step S15d).

[0300]In the MS 2d, the RTO-requirement receiver 201d receives the
above-mentioned control message expressing "RTO acquisition" from the BS
3d via the distributor 251d (step S16d), acquires the present RTO value
from the RTO value storage 252d (step S17d), and requests the RTO value
transmitter 203d to transmit the RTO value while notifying it of the RTO
value (step S18d).

[0301]The RTO value transmitter 203d creates a control message by
specifying the RTO value sent from the RTO-request receiver 201d at the
data portion and specifying a message type (TYPE#04) expressing "RTO
transmission" at the header portion, and transmits that control message
to the distributor 251d (step S19d).

[0302]Upon receiving it, the distributor 251d adds the CID (CID#01) at the
header portion of that control message and transmits that control message
to the BS 3d via the wireless network (step S20d).

[0303]Next, in the BS 3d, the RTO request unit 106d receives the
above-mentioned control message expressing "RTO transmission" (step
S21d), acquires the RTO value specified at the data portion of the
message, and transmits the acquired RTO value to the
line-speed-regulation judgment unit 108d (step S22d).

[0304]The line-speed-regulation judgment unit 108d compares the RTO value
acquired from the RTO request unit 106d with the HO processing time
estimated by the HO time estimator 107d (step S23d). When the RTO value
is larger than the HO processing time (see Y route of step S23d), the
line-speed-regulation judgment unit 108d judges that the line speed
regulation is not necessary and requests the HO-execution-confirmation
notifier 103d to transmit an HO-execution-confirmation notification by
notifying it of the above-mentioned MS identifier (MS#01) (step S28d).

[0305]On the other hand, when the RTO value is equal to or smaller than
the HO processing time (see N route of step S23d), the
line-speed-regulation judgment unit 108d judges that the line speed
regulation (lowering control) is necessary, because the timer will exceed
the RTO value during the HO process without it, and requests the line
speed regulator 102d to regulate (perform control to lower) the line
speed by notifying it of the above-mentioned MS identifier (MS#01).

[0306]Upon the reception of the above-mentioned request for line speed
regulation, the line speed regulator 102d creates a control message in
which, as described earlier for FIG. 5, the symbol number and the channel
number (in the present example, the symbol number and the channel number
lowered at a preset rate (or lowered by a specified range of lowering),
for example) are specified at the data portion and a message type
(TYPE#01) expressing "line speed regulation" and the CID (CID#01) of the
corresponding MS 2d, which is acquired from the MS information storage
152d using the above-mentioned MS identifier (MS#01) as the search key,
are specified at the header portion, and transmits that control message
to the distributor 151d (steps S24d, S25d). Upon receiving it, the
distributor 151d transmits that control message to the MS 2d via the
wireless network (step S26d).

[0307]Subsequently, the line speed regulator 102d notifies the RTO request
unit 106d of a request for RTO acquisition again (step S38d).

[0308]Then, in the MS 2d and the BS 3d, the above-mentioned steps S13d to
S26d and step S38d are carried out repeatedly until the
line-speed-regulation judgment unit 108d of the BS 3d judges that the RTO
value is larger than the HO processing time (see Y route of step S23d)
and the regulation of the line speed is not necessary any more. Upon such
a judgment, the line-speed-regulation judgment unit 108d requests the
HO-execution-confirmation notifier 103d to transmit the
HO-execution-confirmation notification while notifying it of the
above-mentioned MS identifier (MS#01) (step S28d).

[0309]Upon the reception of the above-mentioned request for transmission
of the HO-execution-confirmation notification from the
line-speed-regulation judgment unit 108d or the line speed regulator
102d, the HO-execution-confirmation notifier 103d creates a control
message in which, as described earlier for FIG. 5, the information
expressing "HO execution enabling" is specified at the data portion and a
message type (TYPE#02) expressing "HO execution confirmation" and the CID
(CID#01) of the corresponding MS 2d, which is acquired from the MS
information storage 152d, are specified at the header portion, and
transmits that control message to the distributor 151d (steps S29d,
S30d).

[0310]Upon receiving it, the distributor 151d transmits that control
message to the MS 2d via the wireless network (step S31d).

[0312]Incidentally, the line speed, which was controlled to be lowered by
the aforementioned procedure before executing the HO, is reassigned to
the MS 2d by the HO-target BS 4d independently of the state before the HO
execution after completion of the HO. Concerning the reassignment method
and the line speed, it is possible that, for example, the HO-target BS 4d
determines the line speed to be reassigned to the MS 2d in accordance
with the number of the currently-processed MSs and notifies the MS 2d of
the determined line speed using a control message (TYPE#01) when the MS
2d communicates with the HO-target BS 4d.

[0313]As described above, according to the third embodiment of the present
invention, when an HO execution is decided by the MS 2d and the
requirement for it is sent to the BS 3d, the BS 3d performs control to
lower the line speed of the communication with the MS 2d repeatedly
before executing the HO, which creates a simulated congestion of the
line. This can increase the RTO value to the degree that the RTO value is
longer than the time that is necessary for completion of the HO process.

[0314]Therefore, in addition to the operation and effect of the
aforementioned embodiments, it is possible in the present embodiment that
the RTO value is increased efficiently and appropriately in accordance
with the present communication situation, because the communication speed
at which the BS 3d communicates with the MS 2d via the wireless network
is controlled to be lowered repeatedly until the RTO value becomes longer
than the time required for completion of the HO process at a preset rate
(or by a specified range of lowering).

[C1] Description of the Variation of the Third Embodiment

[0315]In the above-mentioned third embodiment, explanation was given on
the case where the necessities of HO processes are checked and decided by
the MS 2d, in other words, the MS 2d takes the initiative to execute HO
processes. However, the BS 3d may take the initiative to execute HO
processes. Such an embodiment will be described in the explanation of the
present variation example with reference to FIG. 22 to FIG. 24.

[0316]FIG. 22 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
variation of the above-mentioned third embodiment. FIG. 23 is a sequence
diagram illustrating an operation (handover process) of the wireless
communication system (especially, the MS 2e) illustrated in FIG. 22. FIG.
24 is a sequence diagram illustrating an operation (handover process) of
the wireless communication system (especially, the BS 3e) illustrated in
FIG. 22.

[0317]Namely, in the wireless communication system 1e illustrated in FIG.
22, at least one MS 2e and a plurality of BSs 3e (including the HO-target
BS 4e) are connected to each other via a wireless network, in the same
way as the wireless communication system 1d of the third embodiment.
However, it differs from the wireless communication system 1d of the
third embodiment in that the BS 3e decides whether HO processes will be
executed or not.

[0318](Description of MS 2E)

[0319]Therefore, the MS 2e of the present example comprises an adjacent BS
information storage 211e, an adjacent BS information registrar 253e, an
RTO-request receiver 201e, an RTO calculator 202e, an RTO value storage
252e, an RTO value transmitter 203e, an HO-execution-confirmation
receiver 208e, an HO processor 210e, and a distributor 251e, which are of
the same or similar functions as or to each corresponding units 211d,
253d, 201d, 202d, 252d, 203d, 208d, 210d and 251d described earlier for
FIG. 19 to FIG. 21, and it may not need the functions of the HO execution
judgment unit 204d and the HO-target BS decider 206d, which were
illustrated in FIG. 19.

[0320](Description of BS 3E)

[0321]On the other hand, the BS 3e of the present example comprises an RTO
request unit 106e, an HO time estimator 107e, a line-speed-regulation
judgment unit 108e, an adjacent BS information registrar 153e, an
adjacent BS information storage 111e, an MS information storage 152e, a
line speed regulator 102e, an HO-execution-confirmation notifier 103e,
and a distributor 151e, which are of the same or similar functions as or
to each corresponding units described earlier for FIG. 19, and also
comprises an HO execution judgment unit 104e and an HO-target BS decider
105e, in place of the above-mentioned HO-execution-notification receiver
101d described earlier for FIG. 19.

[0322]The HO execution judgment unit 104e of the BS 3e collects
information about the execution condition of HO processes and judges
whether the HO processes will be executed or not. When the HO is
executed, the HO execution judgment unit 104e notifies the HO-target BS
decider 105e of the HO execution and requests the HO-target BS decider
105e to decide the HO-target BS 4e. Namely, the HO execution judgment
unit 104e performs one of the functions of the handover decision means
which autonomously decides execution of HO of the communication with the
MS 2e to another BS 3e (HO-target BS 4e).

[0323]The HO-target BS decider 105e decides the HO-target BS 4e on the
basis of the adjacent BS information stored in the adjacent BS
information storage 111e by selecting from the adjacent BSs 3e that are
already registered, with the reception of the request for decision of the
HO-target BS 4e from the HO execution judgment unit 104e as a trigger.
Concerning the criterion of deciding (selecting) the BS, the BS 3e whose
radio field strength is higher than those of the other BSs 3e can be
selected (decided) as the HO-target BS 4e, for example, also in the
present variation.

[0324](Description of Entire Operation of Wireless Communication System
1e)

[0325]In the following, an example of the entire operation of the wireless
communication system 1e of which construction is such as described above
is illustrated in FIG. 23 and FIG. 24.

[0326]First, the MS 2e acquires the CID (for example, CID#01) of the
connection to the serving BS 3e in advance. The CID (CID#01) is
controlled by the distributor 251e.

[0327]In addition, in the MS 2e, the adjacent BS information registrar
253e acquires adjacent BS information about the BSs 3e (including the
HO-target BS 4e) that are geographically neighboring on the serving BS
3e, and registers it in the adjacent BS information storage 211e (step
S1e), as illustrated in FIG. 23. Moreover, in the MS 2e, the RTO
calculator 202e measures the RTTs of the TCP data that are
transmitted/received to/from the communication parties (namely, other MSs
and Bss 3e) regularly or irregularly and calculates the RTO values from
those RTTs so as to store them in the RTO value storage 252e (step S2e).

[0328]On the other hand, the BS 3e acquires information (MS information)
about the MS 2e, such as the identifier of the MS 2e (for example, MS#01)
and capability types supported by the MS 2e (typically including security
function, packet retransmission function, MIMO function, etc.), and
registers it in the MS information storage 152e. That MS information can
be acquired by, for example, transmission of necessary information also
from the MS 2e at the phase when the MS 2e starts to communicate with the
BS 3e.

[0329]Next, in the BS 3e, the adjacent BS information registrar 153e
acquires the adjacent BS information about the geographically neighboring
BSs 3e (including the HO-target BS 4e) in advance, and registers it in
the adjacent BS information storage 111e (step S3e). That adjacent BS
information can be acquired by, for example, exchanging information with
the neighboring BSs 3e (including the HO-target BS 4e) regularly or
irregularly with the above-mentioned CIDs to be used for exchanging BS
information.

[0330]After the above-mentioned preliminary processes such as registration
of information in the MS 2e and the BS 3e, the BS 3e checks whether an HO
should be executed or not by measuring the radio field strength of the MS
2e regularly or irregularly by the HO execution judgment unit 104e. When
the radio field strength is lower than the value that was preset by the
administrator with a command, the HO execution judgment unit 104e judges
that an HO execution is necessary and requests the HO-target BS decider
106e[sic] to decide the HO-target BS 4e (step S4e).

[0331]The HO-target BS decider 106e[sic] refers to the adjacent BS
information storage 111e, with the above-mentioned request from the HO
execution judgment unit 104e as a trigger, and determines the HO-target
BS 4e on the basis of the adjacent BS information (step S5e). For
example, the BS 3e of which radio field strength is the highest of the
neighboring BSs 3e is determined to be the HO-target BS 4e. Then, the
HO-target BS decider 106e[sic] sends a request for estimation of the HO
processing time to the HO time estimator 107e together with the BS
identifier (BS#02) of the HO-target BS 4e (step S10e). In addition, the
HO-target BS decider 106e[sic] notifies the HO time estimator 107e of the
MS identifier (MS#01) corresponding to the MS 2e of which communication
is going to be handed over.

[0332]The HO time estimator 107e acquires the MS information, from the MS
information storage 152e, about the corresponding MS 2e on the basis of
the above-mentioned MS identifier (MS#01) (step 11e), and transmits
information about the functions supported by the MS 2e to the BS 3e
(HO-target BS 4e) that possesses the above-mentioned BS identifier
(BS#02). In addition, the HO time estimator 107e estimates time (HO
processing time) required for negotiation between the MS 2e and the BS 3e
at the time of HO process on the basis of the information about the
negotiation which is stored in the adjacent BS information storage 111e,
and transmits the estimated HO processing time to the line speed
regulator 108e.

[0333]The RTO request unit 106a creates a control message expressing "RTO
acquisition" when the RTO request unit 106a receives the above-mentioned
RTO-value acquisition request from the HO time estimator 107a (step
S12a). For example, the RTO request unit 106a searches the MS information
storage 152e using the above-mentioned MS identifier (MS#01) as a search
key so as to acquire the CID (CID#01) of the corresponding MS 2a (step
S13a), and specifies that CID at the header portion of the control
message, as well as specifies the message type (TYPE#03) expressing "RTO
acquisition" at the header portion. Then the RTO request unit 106a
transmits the control message to the distributor 151a (step S14a).

[0334]Upon receiving it, the distributor 151e transmits that control
message to the MS 2e via the wireless network (step S15e).

[0335]In the MS 2e, the RTO-requirement receiver 201e receives the
above-mentioned control message expressing "RTO acquisition" from the BS
3e via the distributor 251e (step S16e), acquires the present RTO value
from the RTO value storage 252e (step S17e), and requests the RTO value
transmitter 203e to transmit the RTO value while notifying it of the RTO
value (step S18e).

[0336]The RTO value transmitter 203e creates a control message by
specifying the RTO value sent from the RTO-request receiver 201e at the
data portion and specifying a message type (TYPE#04) expressing "RTO
transmission" at the header portion, and transmits that control message
to the distributor 251e (step S19e).

[0337]Upon receiving it, the distributor 251e adds the CID (CID#O1) at the
header portion of that control message and transmits that control message
to the BS 3e via the wireless network (step S20e).

[0338]Next, in the BS 3e, the RTO request unit 106e receives the
above-mentioned control message expressing "RTO transmission" (step
S21e), acquires the RTO value specified at the data portion of the
message, and transmits the acquired RTO value to the
line-speed-regulation judgment unit 108e (step S22e).

[0339]The line-speed-regulation judgment unit 108e compares the RTO value
acquired from the RTO request unit 106e with the HO processing time
estimated by the HO time estimator 107e (step S23e). When the RTO value
is larger than the HO processing time (refer to Y route of step S23e),
the line-speed-regulation judgment unit 108e judges that the line speed
regulation is not necessary and requests the HO-execution-confirmation
notifier 103e to transmit an HO-execution-confirmation notification by
notifying it of the above-mentioned MS identifier (MS#01) (step S28e).

[0340]On the other hand, when the RTO value is equal to or smaller than
the HO processing time (see N route of step S23e), the
line-speed-regulation judgment unit 108e judges that the line speed
regulation (lowering control) is necessary, because the timer will exceed
the RTO value during the HO process without it, and requests the line
speed regulator 102e to regulate (perform control to lower) the line
speed by notifying it of the above-mentioned MS identifier (MS#01).

[0341]Upon the reception of the above-mentioned request for line speed
regulation, the line speed regulator 102e creates a control message in
which, as described earlier for FIG. 5, the symbol number and the channel
number (in the present example, the symbol number and the channel number
lowered at a preset rate (or lowered by a specified range of lowering),
for example) are specified at the data portion and a message type
(TYPE#01) expressing "line speed regulation" and the CID (CID#01) of the
corresponding MS 2e, which is acquired from the MS information storage
152e using the above-mentioned MS identifier (MS#01) as the search key,
are specified at the header portion, and transmits that control message
to the distributor 151e (steps S24e, S25e).

[0342]Upon receiving it, the distributor 151e transmits that control
message to the MS 2e via the wireless network (step S26e).

[0343]Subsequently, the line speed regulator 102e notifies the RTO request
unit 106e of a request for RTO acquisition again (step S38e).

[0344]Then, in the MS 2e and the BS 3e, the above-mentioned steps S13e to
S26e and step S38e are carried out repeatedly until the
line-speed-regulation judgment unit 108e of the BS 3e judges that the RTO
value is larger than the HO processing time (see Y route of step S23e)
and the regulation of the line speed is not necessary any more. Upon such
a judgment, the line-speed-regulation judgment unit 108e requests the
HO-execution-confirmation notifier 103e to transmit the
HO-execution-confirmation notification while notifying it of the
above-mentioned MS identifier (MS#01) (step S28e).

[0345]Upon the reception of the above-mentioned request for transmission
of the HO-execution-confirmation notification from the
line-speed-regulation judgment unit 108e or the line speed regulation
unit 102e, the HO-execution-confirmation notifier 103e creates a control
message in which, as described earlier for FIG. 5, the information
expressing "HO execution enabling" is specified at the data portion and a
message type (TYPE#02) expressing "HO execution confirmation" and the CID
(CID#01) of the corresponding MS 2e, which is acquired from the MS
information storage 152e, are specified at the header portion, and
transmits that control message to the distributor 151e (steps S29e,
S30e).

[0346]Upon receiving it, the distributor 151e transmits that control
message to the MS 2e via the wireless network (step S31e).

[0348]Incidentally, the line speed, which was controlled to be lowered by
the aforementioned procedure before executing the HO, is reassigned to
the MS 2e by the HO-target BS 4e after completion of the HO independently
of the state before the HO execution. Concerning the reassignment method
and the line speed, it is possible that, for example, the HO-target BS 4e
determines the line speed to be reassigned to the MS 2e in accordance
with the number of the currently-processed MSs and notifies the MS 2e of
the determined line speed using a control message (TYPE#01) when the MS
2e communicates with the HO-target BS 4e.

[0349]In this way, the wireless communication system 1e of the present
variation can also increase the RTO value so as to prevent the timer
(retransmission timer) from exceeding the RTO value during the HO process
by performing control to lower the line speed of the communication with
the MS 2e until the RTO value becomes longer than the time required for
completion of the HO process at a preset rate (or by a specified range of
lowering) before executing the HO, in the same way as the wireless
communication system 1d of the third embodiment. Therefore, it is
possible to inhibit the throughput reduction caused by a slow-start
algorithm originating from the above-mentioned time-out.

[D] Description of the Fourth Embodiment

[0350]FIG. 25 is a block diagram illustrating the construction of the
substantial part of the wireless communication system according to the
fourth embodiment of the present invention. FIG. 26 is a sequence diagram
illustrating an operation (handover process) of the wireless
communication system (especially, MS 2f) illustrated in FIG. 25. FIG. 27
is a sequence diagram illustrating an operation (handover process) of the
wireless communication system (especially, BS 3f) illustrated in FIG. 25.
All of these figures illustrate an embodiment in which the MS judges
whether HO processes are executed or not, in the same way as the first to
third embodiments.

[0351]Therefore, also in the wireless communication system 1f illustrated
in FIG. 25, at least one MS 2f and a plurality of BSs 3f (including the
HO-target BS 4f) are connected to each other via a wireless network.
However, the present embodiment differs from the construction illustrated
in FIG. 1 mainly in that an HO-execution-time decider 205f and an HO
start judgment unit 209f are added to the MS 2f.

[0352]Therefore, the MS 2f comprises an HO execution judgment unit 204f,
an HO-target BS decider 206f, an adjacent BS information storage 211f, an
adjacent BS information registrar 253f, an HO-execution-notification
transmitter 207f, an RTO-request receiver 201f, an RTO calculator 202f,
an RTO value storage 252f, an RTO value transmitter 203f, an
HO-execution-confirmation receiver 208f, an HO processor 210f, and a
distributor 251f, which are of the same or similar functions as or to
each corresponding units 204, 206, 211, 253, 207, 201, 202, 252, 203,
208, 210 and 251 described earlier for FIG. 1, and also comprises an
HO-execution-time decider 205f and an HO start judgment unit 209f.

[0353]On the other hand, the BS 3f comprises an HO-execution-notification
receiver 101f, an HO time estimator 107f, an RTO request unit 106f, a
line-speed-regulation judgment unit 108f, a line speed regulator 102f, an
MS information storage 152f, an adjacent BS information registrar 153f,
an adjacent BS information storage 111f, an HO-execution-confirmation
notifier 103f, and a distributor 151f, which are of the same or similar
functions as or to each corresponding units 101, 106, 107, 108, 102, 152,
153, 111, 103 and 151 described earlier for FIG. 1.

[0354](Description of MS 2F)

[0355]In the MS 2f, the HO execution judgment unit 204f, the HO-target BS
decider 206f, the adjacent BS information storage 211f, the adjacent BS
information registrar 253f, the HO-execution-notification transmitter
207f, the RTO-request receiver 201f, the RTO calculator 202f, the RTO
value storage 252f, the RTO value transmitter 203f, the
HO-execution-confirmation receiver 208f, the HO processor 210f, and the
distributor 251b[sic] are of the same or similar functions as or to each
corresponding units described earlier for FIG. 1.

[0356]The HO execution judgment unit 204f collects information about the
execution condition of HO processes and judges whether the HO will be
executed or not, in the same way as the HO execution judgment unit 204.
In the present example, the HO execution judgment unit 204f notifies the
HO-execution-time decider 205f of the HO execution.

[0357]The HO-execution-time decider 205f decides the HO execution time on
the basis of the information about the HO execution time preset by the
user, with the reception of the notification of the HO execution from the
HO execution judgment unit 204f as a trigger, and notifies the HO start
judgment unit 209f of that HO execution time. Therefore, the
HO-execution-time decider 205f performs the function of the
handover-execution-time decision means which decides the execution time
of the HO process when the above-mentioned HO execution judgment unit
204f judges that an HO process will be executed.

[0358]The HO-execution-confirmation receiver (enabling-acknowledgement
reception means) 208f receives, from the BS 3f, an
HO-execution-confirmation notification (enabling acknowledgement) with
respect to the HO execution notification (execution request) sent from
the HO-execution-notification transmitter 207f. In the present example,
the HO-execution-confirmation receiver 208f notifies the HO start
judgment unit 209f of the HO execution confirmation, with the reception
of the above-mentioned HO-execution-confirmation notification as a
trigger.

[0359]The HO start judgment unit 209f requests the HO processor 210f to
execute the HO process when the HO-execution-confirmation notification is
sent from the HO-execution-confirmation receiver 208f by the HO execution
time, which was reported by the HO-execution-time decider 205f. On the
other hand, when the HO-execution-confirmation notification is not sent
from the HO-execution-confirmation receiver 208f by the HO execution
time, the HO start judgment unit 209f does not request the HO processor
210f to execute the HO process. Namely, the HO start judgment unit 209f
postpones or quits the HO execution when the HO-execution-confirmation
notification is not sent from the HO-execution-confirmation receiver 208f
by the above-mentioned HO execution time.

[0360]Therefore, the HO start judgment unit 209f performs the function of
the handover-execution control means which postpones the handover
execution until after the HO execution time decided by the
HO-execution-time decider 205f when the HO-execution-confirmation
notification (enabling acknowledgement) is not received by the
HO-execution-confirmation receiver 208f within a predetermined period of
time (the HO execution time reported by the HO-execution-time decider
205f) after the transmission of the HO execution request by the
HO-execution-notification transmitter 207f.

[0361]The HO processor 210f executes an HO process, with which the
communication process is handed over from the serving BS 3f to the
HO-target BS 4f, upon reception of the HO execution request from the HO
start judgment unit 209f.

[0362]With the above-mentioned structure, the MS 2f according to the
fourth embodiment of the present invention can decide an HO execution and
request the BS 3f to perform an HO process upon reception of a signal
from the BS 3f, as well as can transmit the RTO value which is calculated
by the RTO calculator 202f to the BS 3 in accordance with a request from
the BS 3f.

[0363]In addition, the MS 2f can perform congestion control (communication
speed control) to lower the line speed of the communication with the
communication party upon time-out of the RTO value. Moreover, in the
present example, the HO start judgment unit 209f of the MS 2f can quit or
postpone the HO execution when the HO-execution-confirmation notification
can not be received by the predetermined time specified by the
HO-execution-time decider 205f.

[0364](Description of BS 3F)

[0365]On the other hand, in the BS 3f, the HO-execution-notification
receiver 101f, the RTO request unit 106f, the HO time estimator 107f, the
line-speed-regulation judgment unit 108f, the adjacent BS information
registrar 153f, the adjacent BS information storage 111f, the MS
information storage 152f, the line speed regulator 102f, the
HO-execution-confirmation notifier 103f, and the distributor 151f are of
the same or similar functions as or to each corresponding units described
earlier for FIG. 1.

[0366]Therefore, the BS 3f according to the fourth embodiment of the
present invention estimates time required for an HO process upon
receiving an HO execution notification from the MS 2f, compares the
estimated result and the present RTO value which was acquired by sending
a request to the MS 2f, regulates (performs control to lower) the line
speed on the basis of the comparison result, and then executes the HO, in
the same way as the BS 2[sic] of the first embodiment. In this way, the
BS 3f can execute an HO process with the RTO value increased in order for
the timer not to exceed the RTO value by creating simulated line
congestion through a control to lower the speed of the communication with
the MS 2f, if it is necessary.

[0367](Description of Entire Operation of Wireless Communication System
1f)

[0368]Next, an example of the entire operation of the wireless
communication system 1f having the above-mentioned structure will be
described with reference to FIG. 26 and FIG. 27.

[0369]First, the MS 2f acquires the CID (for example, CID#01) of the
connection to the serving BS 3f in advance. The CID (CID#01) is
controlled by the distributor 251f.

[0370]In addition, in the MS 2f, the adjacent BS information registrar
253f acquires information (adjacent BS information) about the BSs 3f
(including the HO-target BS 4f) that are geographically neighboring on
the serving BS 3f, such as their frequencies used, capability types
supported (typically including security function, packet retransmission
function, MIMO function, etc.), radio field strengths, and the number of
MSs in charge, and registers it in the adjacent BS information storage
211f (step S1f). The MS 2f can acquire that adjacent BS information by,
for example, receiving it when it is transmitted (reported) from the BS
3f.

[0371]Moreover, in the MS 2f, the RTO calculator 202f measures the RTTs of
the TCP data that are transmitted/received to/from the communication
parties (namely, other MSs and Bss 3f) regularly or irregularly and
calculates the RTO values from those RTTs so as to store them in the RTO
value storage 252f (step S2f).

[0372]On the other hand, the BS 3f acquires information (MS information)
about the MS 2f such as the identifier of the MS 2f (for example, MS#01)
and capability types supported by the MS 2f (typically including security
function, packet retransmission function, MIMO function, etc.), and
registers it in the MS information storage 152f. That MS information can
be acquired by, for example, transmission of necessary information also
from the MS 2f at the phase when the MS 2f starts to communicate with the
BS 3f.

[0373]Next, in the BS 3f, the adjacent BS information registrar 153f
acquires information (adjacent BS information) about the geographically
neighboring BSs 3f (including the HO-target BS 4f) in advance, such as
the frequency used and the capability types supported, and registers it
in the adjacent BS information storage 111f (step S3f). That adjacent BS
information can be acquired by, for example, exchanging information with
the neighboring BSs 3f (including the HO-target BS 4f) regularly or
irregularly with the above-mentioned CIDs to be used for exchanging BS
information.

[0374]After the above-mentioned preliminary processes such as registration
of information in the MS 2f and the BS 3f, the MS 2f checks whether an HO
should be executed or not by measuring the radio field strength of the
serving BS 3f regularly or irregularly by the HO execution judgment unit
204f. When the radio field strength is lower than the value that was
preset by the administrator with a command, the HO execution judgment
unit 204f judges that an HO execution is necessary and requests the
HO-target BS decider 206f to decide the HO-target BS 4f (step S4f), as
well as request the HO-execution-time decider 205f to decide the HO
execution time (step S39f).

[0375]Then, in the MS 2f, the HO-execution-time decider 205f decides the
HO execution time on the basis of the information about the HO execution
time preset by the user, upon reception of the notification of the HO
execution from the HO execution judgment unit 204f, and notifies the HO
start judgment unit 209f of that HO execution time (step S40f).

[0376]The HO-target BS decider 206f refers to the adjacent BS information
storage 211f, with the above-mentioned request from the HO execution
judgment unit 204f as a trigger, and determines the HO-target BS 4f on
the basis of the adjacent BS information (step S5f). For example, the BS
3f of which radio field strength is the highest of the neighboring BSs 3f
is determined to be the HO-target BS 4f. Then, the HO-target BS decider
206f notifies the HO-execution-notification transmitter 207f of the BS
identifier (for example, BS#02) of the determined HO-target BS 4f, and
also, requests the HO-execution-notification transmitter 207f to transmit
an HO execution notification (step S6f).

[0377]The HO-execution-notification transmitter 207f creates a control
message, upon reception of the above-mentioned request from the HO-target
BS decider 206f, in which the message type (TYPE#00) expressing an "HO
execution notification" is specified at the header portion and the BS
identifier (BS#02) of the HO-target BS 4f and the MS identifier (MS#01)
of the MS 2f, of which communication process is going to be handed over,
are specified at the data portion and transmits the control message to
the distributor 251f (step S7f).

[0378]The distributor 251f specifies the CID (CID#01) at the header
portion of that control message and transmits that control message to the
BS 3f via the wireless network (step S8f).

[0379]In the BS 3[sic], the HO-execution-notification receiver 101f
receives that control message (HO execution notification) via the
distributor 151f, which is transmitted from the MS 2f (step S9f),
acquires the MS identifier (MS#O1) and the BS identifier (BS#02)
contained in the data portion of that HO execution notification, and then
sends a request for estimation of the HO processing time to the HO time
estimator 107f together with the BS identifier (step S10f). In addition,
the HO-execution-notification receiver 101f acquires the MS identifier
(MS#01) from the data portion of that control message and notifies the HO
time estimator 107f of the MS identifier (MS#01).

[0380]The HO time estimator 107f acquires the MS information, from the MS
information storage 152f, about the MS 2f that transmitted the HO
execution notification on the basis of the above-mentioned MS identifier
(MS#01) (step S11f), and transmits information about the functions
supported by the MS 2f to the BS 3f (HO-target BS 4f) of which identifier
is the above-mentioned BS identifier (BS#02). In addition, the HO time
estimator 107f estimates time (HO processing time) required for
negotiation between the MS 2f and the BS 3f at the time of HO process on
the basis of the information about the negotiation which is stored in the
adjacent BS information storage 111f, and transmits the estimated HO
processing time to the line speed regulator 108f. Moreover, the HO time
estimator 107f notifies the RTO request unit 106f of an RTO acquisition
request in order to acquire the RTO value of the MS 2f corresponding to
the MS identifier that is acquired from the HO-execution-notification
receiver 101f (step S12f).

[0381]The RTO request unit 106f creates a control message expressing "RTO
acquisition" when the RTO request unit 106f receives the above-mentioned
RTO-value acquisition request from the HO time estimator 107f. For
example, the RTO request unit 106f searches the MS information storage
152f using the above-mentioned MS identifier (MS#01) as a search key so
as to acquire the CID (CID#01) of the corresponding MS 2f (step S13f),
and specifies that CID at the header portion of the control message, as
well as specifies a message type (TYPE#03) expressing "RTO acquisition"
at the header portion. Then the RTO request unit 106f transmits the
control message to the distributor 151f (step S14f).

[0382]Upon receiving it, the distributor 151f transmits that control
message to the MS 2f via the wireless network (step S15f).

[0383]In the MS 2f, the RTO-requirement receiver 201f receives the
above-mentioned control message expressing "RTO acquisition" from the BS
3f via the distributor 251f (step S16f), acquires the present RTO value
from the RTO value storage 252f (step S17f), and requests the RTO value
transmitter 203f to transmit the RTO value while notifying it of the RTO
value (step S18f).

[0384]The RTO value transmitter 203f creates a control message by
specifying the RTO value sent from the RTO-requirement receiver 201f at
the data portion and specifying a message type (TYPE#04) expressing "RTO
transmission" at the header portion, and transmits that control message
to the distributor 251f (step S19f).

[0385]Upon receiving it, the distributor 251f adds the CID (CID#01) at the
header portion of that control message and transmits that control message
to the BS 3f via the wireless network (step S20f).

[0386]Next, in the BS 3f, the RTO request unit 106f receives the
above-mentioned control message expressing "RTO transmission" (step
S21f), acquires the RTO value specified at the data portion of the
message, and transmits the acquired RTO value to the
line-speed-regulation judgment unit 108f (step S22f).

[0387]The line-speed-regulation judgment unit 108f compares the RTO value
acquired from the RTO request unit 106f with the HO processing time
estimated by the HO time estimator 107f (step S23f). When the RTO value
is larger than the HO processing time (see Y route of step S23f), the
line-speed-regulation judgment unit 108f judges that the line speed
regulation is not necessary and requests the HO-execution-confirmation
notifier 103f to transmit an HO-execution-confirmation notification by
notifying it of the above-mentioned MS identifier (MS#01) (step S28f).

[0388]On the other hand, when the RTO value is equal to or smaller than
the HO processing time (see N route of step S23f), the
line-speed-regulation judgment unit 108f judges that the line speed
regulation (lowering control) is necessary, because the timer will exceed
the RTO value during the HO process without it, and requests the line
speed regulator 102f to regulate (perform control to lower) the line
speed by notifying it of the above-mentioned MS identifier (MS#01).

[0389]Upon the reception of the above-mentioned request for line speed
regulation, the line speed regulator 102f creates a control message in
which, as described earlier for FIG. 5, the symbol number and channel
number (both of which are "1" in the present example, for example) are
specified at the data portion and a message type (TYPE#01) expressing
"line speed regulation" and the CID (CID#01) of the corresponding MS 2f,
which is acquired from the MS information storage 152f using the
above-mentioned MS identifier (MS#01) as the search key, are specified at
the header portion, and transmits that control message to the distributor
151f (step S24f, S25f).

[0390]Upon receiving it, the distributor 151f transmits that control
message to the MS 2f via the wireless network (step S26f).

[0391]Subsequently, the line speed regulator 102f requests the
HO-execution-confirmation notifier 103f to transmit an
HO-execution-confirmation notification by notifying it of the
above-mentioned MS identifier (MS#01) (step S27f).

[0392]Upon the reception of the above-mentioned request for transmission
of the HO-execution-confirmation notification from the
line-speed-regulation judgment unit 108f or the line speed regulator
102f, the HO-execution-confirmation notifier 103f creates a control
message in which, as described earlier for FIG. 5, the information
expressing "HO execution enabling" is specified at the data portion and a
message type (TYPE#02) expressing "HO execution confirmation" and the CID
(CID#01) of the corresponding MS 2f, which is acquired from the MS
information storage 152f, are specified at the header portion, and
transmits that control message to the distributor 151f (steps S29f,
S30f).

[0393]Upon receiving it, the distributor 151f transmits that control
message to the MS 2f via the wireless network (step S31f).

[0394]Next, in the MS 2f, the HO-execution-confirmation receiver 208f
receives the above-mentioned control message expressing "HO execution
confirmation" via the distributor 251f (step S32f), and then the HO start
judgment unit 209f judges whether the current time is the HO execution
time reported by the HO-execution-time decider 205f or not (step S41f).
When judged that the current time is the HO execution time (see Y route
of step S41f), the HO start judgment unit 209f then judges whether the
above-mentioned control message expressing "HO execution confirmation" is
received or not (step S42f). When judged that the control message is
received (see Y route of step S42f), the HO start judgment unit 209f
requests the HO processor 210f to perform HO process (step S33f), and
thereby, the HO process to the HO-target BS 4f is executed.

[0395]On the other hand, when judged, by the HO start judgment unit 209f,
that the current time is not the HO execution time reported by the
HO-execution-time decider 205f (see N route of step S41f), or that the
above-mentioned control message expressing "HO execution confirmation" is
not received (see N route of step S42f), the HO execution will be quitted
(or postponed).

[0396]Incidentally, the line speed, which was controlled to be lowered by
the aforementioned procedure before executing the HO, is reassigned to
the MS 2f by the HO-target BS 4f independently of the state before the HO
execution after completion of the HO. Concerning the reassignment method
and the line speed, it is possible that, for example, the HO-target BS 4f
determines the line speed to be reassigned to the MS 2f in accordance
with the number of the currently-processed MSs and notifies the MS 2f of
the determined line speed using a control message (TYPE#01) when the MS
2f communicates with the HO-target BS 4f.

[0397]As described above, according to the fourth embodiment of the
present invention, when an HO execution is decided by the MS 2f and the
requirement for it is sent to the BS 3f, the BS 3f performs control to
lower the line speed of the communication with the MS 2f before executing
the HO, which causes simulated congestion of the line. This can achieve
the same operation and effect as those of the aforementioned embodiments.
In addition, when the MS 2f can not receive the HO enabling
acknowledgement from the BS 3f with respect to the HO execution request
within a predetermined period of time, the HO process will be quitted or
postponed. Therefore, it is possible to prevent the MS 2f from executing
an HO process with the above-mentioned timer value remaining small, even
when the BS 3f can not increase the above-mentioned timer value
sufficiently within a predetermined period of time, namely when the BS 3f
can not create simulated line congestion. This can inhibit the
above-mentioned throughput reduction.

[E] Others

[0398]The embodiments and the respective variations of the present
invention have been explained in detail above. However, it is to be
understood that the embodiments and the respective variations illustrated
above are by no means restrictive and any modifications can be added
thereto insofar as they do not depart from the scope of the present
invention.

[0399]For example, in a wireless communication system whose MS judges
whether HO processes will be executed or not, the BS may include at least
the aforementioned HO-execution-notification receiver 101, line speed
regulator 102, HO-execution-confirmation notifier 103, MS information
storage 152, and distributor 151, as illustrated in FIG. 28. With such a
construction, the RTO value can be increased in order to inhibit the
time-out of the retransmission timer by performing control to lower the
line speed (communication speed) at which the BS communicates with the MS
so as to create a simulated line congestion, with the HO execution
requirement sent to the BS as a trigger.

[0400]In addition, in a mobile communication system whose BS judges
whether HO processes will be executed or not, the BS may include at least
the aforementioned HO execution judgment unit 104, HO-target BS decider
105, adjacent BS information registrar 153, adjacent BS information
storage 111, line speed regulator 102, HO-execution-confirmation notifier
103, MS information storage 152, and distributor 151, as illustrated in
FIG. 29. Such a construction can also achieve the same operation and
effect as those described above.

[0401]According to the above-mentioned embodiments, at least one of the
following effects and advantages can be achieved.

[0402](1) When an execution of a handover process is decided, the line
speed (communication speed) at which the wireless base station
communicates with the wireless terminal is controlled to be lowered so as
to cause a simulated congestion of the line. This can increase the
above-mentioned timer value to the degree that it is longer than the time
that is necessary for completion of the handover process. Therefore, the
time-out of the above-mentioned timer value at the time of a handover
process can be prevented, which makes it possible to inhibit the
throughput reduction caused by the above-mentioned slow-start algorithm
without inducing throughput reduction in normal times.

[0403](2) In addition, in the case where the wireless base station
receives the handover-execution request transmitted from the wireless
terminal, decides the handover execution, and transmits the enabling
acknowledgement of the handover to the wireless terminal so as to execute
the handover, the throughput reduction caused by the above-mentioned
slow-start algorithm can be inhibited even when the wireless terminal
takes the initiative to execute handover processes.

[0404](3) Moreover, in the case where the wireless base station judges
whether the handover can be executed or not on the basis of the
information about the execution condition of the handover, decides the
handover execution when judging that it can be executed, and transmits
the enabling acknowledgement of the handover to the wireless terminal, it
is possible that the handover process is carried out just when it is
necessary.

[0405](4) Moreover, in the case where the wireless base station acquires
the present timer value from the wireless terminal, estimates the time
required for the handover, and performs control to lower the
communication speed on the condition that the present timer value is
smaller than the estimated time at the time of the handover-execution
decision, it is possible that the lowering control can be performed just
when it is necessary, which makes it possible to inhibit the throughput
reduction efficiently.

[0406](5) Moreover, in the case where the wireless base station records
history information about the communication speed at the time of
generation of the timer value or acquisition thereof from the wireless
terminal, calculates the lowering value of the communication-speed
lowering control on the basis of the history information, and performs
control to lower the communication speed by the lowering value, it is
possible that the lowering control is performed on the basis of the
history information, which makes it possible to increase the timer value
necessarily and sufficiently.

[0407](6) Moreover, in the case where the wireless base station reacquires
the present timer value from the wireless terminal while performing
control to lower the communication speed, and performs control to lower
the communication speed until the estimated time falls below the
reacquired, present timer value, it is possible that the lowering control
is performed efficiently in accordance with the present communication
situation, which makes it possible to increase the timer value more
appropriately.

[0408](7) Moreover, in the case where the wireless terminal postpones the
handover when the wireless terminal does not receive the enabling
acknowledgement of the handover from the wireless base station with
respect to the handover-execution request within a predetermined period
of time, it is possible to prevent the handover execution with the timer
value remaining small when the wireless base station can not increase the
timer value sufficiently within a predetermined period of time, which
makes it possible to inhibit the throughput reduction.

INDUSTRIAL APPLICABILITY

[0409]As described above in detail, according to the present invention, it
is possible to prevent the timer from exceeding the timer value during a
handover process by performing, before the handover process, control to
lower the line speed (communication speed) at which the wireless base
station communicates with the wireless terminal so as to create a
simulated line congestion. This makes it possible to control the
throughput reduction caused by the above-mentioned slow-start algorithm.
Therefore, the present invention can be applied to the area of wireless
communication technology, and particularly to the area of mobile wireless
communication technology having a handover function highly effectively.